x86 32-bit machine code, 62 bytes

33 c0 60 8b f2 33 db 33 d2 80 f9 02 72 2e ad 50
0f ab c2 72 26 3b c3 77 01 93 2b c3 d1 e8 72 15
48 68 92 08 0e 02 0f a3 1c 04 5f 73 08 03 d8 43
0f a3 da 73 06 5b e2 d6 61 40 c3 58 61 c3

bt byte ptr [esp + eax], ebx;

    xor eax, eax;   // prepare to return false
    pushad;         // save all registers
    mov esi, edx;   // esi = pointer to input list
    xor ebx, ebx;   // ebx = previously encountered number = 0
    xor edx, edx;   // edx = bitmap of visited numbers = 0

    cmp cl, 2;      // is input list too short?
    jb bad_no_pop;  // bad!

again:
    lodsd;          // read one number
    push eax;

    bts edx, eax;   // check and update the bitmap
    jc bad;         // same number twice? - bad!

    cmp eax, ebx;   // sort two recent numbers (ebx = minimum)
    ja skip1;
    xchg eax, ebx;
skip1:

    // Check whether the line crosses a node
    sub eax, ebx;   // calculate half the difference
    shr eax, 1;
    jc skip_cross;  // odd difference? - no node in the middle
    dec eax;

    push 0x020e0892;// push magic bitmap onto stack
    bt byte ptr [esp + eax], ebx; // is there a node in the middle?
    pop edi;
    jnc skip_cross; // no - skip the check

    add ebx, eax;   // calculate the node in the middle
    inc ebx;
    bt edx, ebx;    // was it visited?
    jnc bad;        // no - bad!

skip_cross:
    pop ebx;
    loop again;

    // The loop was finished normally - return true
    popad;          // restore registers
    inc eax;        // change 0 to 1
    ret;            // return

    // Return false
bad:
    pop eax;        // discard data on stack
bad_no_pop:
    popad;          // restore registers
    ret;            // return

x86 32-bit machine code, 62 60 bytes

33 c0 60 8b f2 33 db 99 80 f9 02 72 2d ad 50 0f
ab c2 72 25 3b c3 77 01 93 2b c3 d1 e8 72 14 68 
92 08 0e 02 0f a3 5c 04 ff 5f 73 07 03 d8 0f a3 
da 73 06 5b e2 d7 61 40 c3 58 61 c3

bt byte ptr [esp + eax - 1], ebx; // -1 because half-difference is 1-based

    xor eax, eax;   // prepare to return false
    pushad;         // save all registers
    mov esi, edx;   // esi = pointer to input list
    xor ebx, ebx;   // ebx = previously encountered number = 0
    cdq;            // edx = bitmap of visited numbers = 0

    cmp cl, 2;      // is input list too short?
    jb bad_no_pop;  // bad!

again:
    lodsd;          // read one number
    push eax;

    bts edx, eax;   // check and update the bitmap
    jc bad;         // same number twice? - bad!

    cmp eax, ebx;   // sort two recent numbers (ebx = minimum)
    ja skip1;
    xchg eax, ebx;
skip1:

    // Check whether the line crosses a node
    sub eax, ebx;   // calculate half the difference
    shr eax, 1;
    jc skip_cross;  // odd difference? - no node in the middle

    push 0x020e0892;// push magic bitmap onto stack
    bt byte ptr [esp + eax - 1], ebx; // is there a node in the middle?
    pop edi;
    jnc skip_cross; // no - skip the check

    add ebx, eax;   // calculate the node in the middle
    bt edx, ebx;    // was it visited?
    jnc bad;        // no - bad!

skip_cross:
    pop ebx;
    loop again;

    // The loop was finished normally - return true
    popad;          // restore registers
    inc eax;        // change 0 to 1
    ret;            // return

    // Return false
bad:
    pop eax;        // discard data on stack
bad_no_pop:
    popad;          // restore registers
    ret;            // return

bt byte ptr [esp + eax], ebx;

    xor eax, eax;   // prepare to return false
    pushad;         // save all registers
    mov esi, edx;   // esi = pointer to input list
    xor ebx, ebx;   // ebx = previously encountered number = 0
    xor edx, edx;   // edx = bitmap of visited numbers = 0

    cmp cl, 2;      // is input list too short?
    jb bad_no_pop;  // bad!

again:
    lodsd;          // read one number
    push eax;

    bts edx, eax;   // check and update the bitmap
    jc bad;         // same number twice? - bad!

    cmp eax, ebx;   // sort two recent numbers (ebx = minimum)
    ja skip1;
    xchg eax, ebx;
skip1:

    // Check whether the line crosses a node
    sub eax, ebx;   // calculate half the difference
    shr eax, 1;
    jc skip_cross;  // odd difference? - no node in the middle
    dec eax;

    push 0x020e0892;// push magic bitmap onto stack
    bt byte ptr [esp + eax], ebx; // is there a node in the middle?
    pop edi;
    jnc skip_cross; // no - skip the check

    add ebx, eax;   // calculate the node in the middle
    inc ebx;
    bt edx, ebx;    // was it visited?
    jnc bad;        // no - bad!

skip_cross:
    pop ebx;
    loop again;

    // The loop was finished normally - return true
    popad;          // restore registers
    inc eax;        // change 0 to 1
    ret;            // return

    // Return false
bad:
    pop eax;        // discard data on stack
bad_no_pop:
    popad;          // restore registers
    ret;            // return

bt byte ptr [esp + eax], ebx;

    xor eax, eax;   // prepare to return false
    pushad;         // save all registers
    mov esi, edx;   // esi = pointer to input list
    xor ebx, ebx;   // ebx = previously encountered number = 0
    xor edx, edx;   // edx = bitmap of visited numbers = 0

    cmp cl, 2;      // is input list too short?
    jb bad_no_pop;  // bad!

again:
    lodsd;          // read one number
    push eax;

    bts edx, eax;   // check and update the bitmap
    jc bad;         // same number twice? - bad!

    cmp eax, ebx;   // sort two recent numbers (ebx = minimum)
    ja skip1;
    xchg eax, ebx;
skip1:

    // Check whether the line crosses a node
    sub eax, ebx;   // calculate half the difference
    shr eax, 1;
    jc skip_cross;  // odd difference? - no node in the middle
    dec eax;

    push 0x020e0892;// push magic bitmap onto stack
    bt byte ptr [esp + eax], ebx; // is there a node in the middle?
    pop edi;
    jnc skip_cross; // no - skip the check

    add ebx, eax;   // calculate the node in the middle
    inc ebx;
    bt edx, ebx;    // was it visited?
    jnc bad;        // no - bad!

skip_cross:
    pop ebx;
    loop again;

    // The loop was finished normally - return true
    popad;          // restore registers
    inc eax;        // change 0 to 1
    ret;            // return

    // Return false
bad:
    pop eax;        // discard data on stack
bad_no_pop:
    popad;          // restore registers
    ret;            // return

x86 32-bit machine code, 64 bytes

33 c0 60 8b f2 33 db 33 d2 80 f9 02 72 30 ad 50
0f ab c2 72 28 3b c3 77 01 93 2b c3 d1 e8 72 17
48 78 1a 68 92 08 0e 02 0f a3 1c 04 5f 73 08 03
 d8 43 0f a3 da 73 06 5b e2 d4 61 40 c3 58 61 c3

    xor eax, eax;   // prepare to return false
    pushad;         // save all registers
    mov esi, edx;   // esi = pointer to input list
    xor ebx, ebx;   // ebx = previously encountered number = 0
    xor edx, edx;   // edx = bitmap of visited numbers = 0

    cmp cl, 2;      // is input list too short?
    jb bad_no_pop;  // bad!

again:
    lodsd;          // read one number
    push eax;

    bts edx, eax;   // check and update the bitmap
    jc bad;         // same number twice? - bad!

    cmp eax, ebx;   // sort two recent numbers (ebx = minimum)
    ja skip1;
    xchg eax, ebx;
skip1:

    // Check whether the line crosses a node
    sub eax, ebx;   // calculate half the difference
    shr eax, 1;
    jc skip_cross;  // odd difference? - no node in the middle
    dec eax;
    js bad;         // same number twice? - bad!

    push 0x020e0892;// push magic bitmap onto stack
    bt byte ptr [esp + eax], ebx; // is there a node in the middle?
    pop edi;
    jnc skip_cross; // no - skip the check

    add ebx, eax;   // calculate the node in the middle
    inc ebx;
    bt edx, ebx;    // was it visited?
    jnc bad;        // no - bad!

skip_cross:
    pop ebx;
    loop again;

    // The loop was finished normally - return true
    popad;          // restore registers
    inc eax;        // change 0 to 1
    ret;            // return

    // Return false
bad:
    pop eax;        // discard data on stack
bad_no_pop:
    popad;          // restore registers
    ret;            // return

x86 32-bit machine code, 62 bytes

33 c0 60 8b f2 33 db 33 d2 80 f9 02 72 2e ad 50
0f ab c2 72 26 3b c3 77 01 93 2b c3 d1 e8 72 15
48 68 92 08 0e 02 0f a3 1c 04 5f 73 08 03 d8 43 
0f a3 da 73 06 5b e2 d6 61 40 c3 58 61 c3

    xor eax, eax;   // prepare to return false
    pushad;         // save all registers
    mov esi, edx;   // esi = pointer to input list
    xor ebx, ebx;   // ebx = previously encountered number = 0
    xor edx, edx;   // edx = bitmap of visited numbers = 0

    cmp cl, 2;      // is input list too short?
    jb bad_no_pop;  // bad!

again:
    lodsd;          // read one number
    push eax;

    bts edx, eax;   // check and update the bitmap
    jc bad;         // same number twice? - bad!

    cmp eax, ebx;   // sort two recent numbers (ebx = minimum)
    ja skip1;
    xchg eax, ebx;
skip1:

    // Check whether the line crosses a node
    sub eax, ebx;   // calculate half the difference
    shr eax, 1;
    jc skip_cross;  // odd difference? - no node in the middle
    dec eax;

    push 0x020e0892;// push magic bitmap onto stack
    bt byte ptr [esp + eax], ebx; // is there a node in the middle?
    pop edi;
    jnc skip_cross; // no - skip the check

    add ebx, eax;   // calculate the node in the middle
    inc ebx;
    bt edx, ebx;    // was it visited?
    jnc bad;        // no - bad!

skip_cross:
    pop ebx;
    loop again;

    // The loop was finished normally - return true
    popad;          // restore registers
    inc eax;        // change 0 to 1
    ret;            // return

    // Return false
bad:
    pop eax;        // discard data on stack
bad_no_pop:
    popad;          // restore registers
    ret;            // return