Product Version: NetBeans IDE Dev (Build 200905190001)
Java: 1.6.0_14-ea; Java HotSpot(TM) Client VM 14.0-b12
System: Windows Vista version 6.0 running on x86; Cp1252; en_US (nb)

have somewhere in the code a line
--------------------
int a = 0;
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select the line and invoke refactor|introduce method, confirm the dialog (with some name)
the resultant code will be like
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int a = foo();
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and a new method will be in the class
--------------------
int foo(){
    int a = 0;
    return a;
}
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now select the line 'int a = foo();' and invoke refactor|introduce method. In the dialog insert the same name as before 
(in this case 'foo') and confirm.
No warning is shown, and the code is now like
--------------------
int foo(){
    int a = 0;
    return a;
}

int foo(){
    int a = foo();
    return a;
}

...
int a = foo()
...
--------------------