Non-idempotent Python

def method():
    if 'a' not in vars():a=0
    a+=1
    if 'a' not in vars():a=0
    a+=1
    print(a)

Initializes the variable a to 0 only if it's not already initialized in the variables table. Then, increments it.

More briefly (thanks to histocrat for len):

def method():
    a=len(vars())+1
    a=len(vars())+1
    print(a)

If the two copies of X could be on the same line, we could do

a=0;a+=1;a

which doubles to

a=0;a+=1;aa=0;a+=1;a

with the "sacrificial lamb" aa eating up the second variable assignment.

Python

Thought of this solution, since try and except was the first way I thought of to determine if a variable existed yet or not.

def method():
    try:a+=1
    except:a=1
    print(a)

Python 2

def method():
    exec'';locals()['a']=locals().get('a',0)+1
    exec'';locals()['a']=locals().get('a',0)+1
    print a

method()

Basically, when exec is encountered in Python 2, it causes a special flag (0x01) to be removed from method.func_code.co_flags, which makes locals assignments have an effect. I exploited this to implement nonlocal support in Python 2 (see line 43 for the xor that modifies the flag).