Steps to reproduce:
1. Start IDLE.
2. Enter some text:

spam
eggs

3. Select the first line ("spam") and press control+c to copy.
4. Select the second line ("eggs") and press control+v to paste.

Expected result:

spam
spam

(With the caret at the end of the second line and no selection.)

Actual result (IDLE 2.6, Tk/Tcl 8.4, Linux):

spam
eggsspam

(With "eggs" selected and the caret at the end of the second line.)

I don't think this happens to me on Windows. It might be deliberate; I
know that Tk has slightly different behaviour on Windows and Linux. But
it doesn't seem to be consistent with other behaviour: for example, if
instead of pasting I start typing, the selected text is erased and the
typed text replaces it.

I can look into this myself, but I would appreciate if someone else can
confirm that this is not the expected behaviour, since perhaps it is
only my Windows background that leads me to expect this.