Almost all Lazarus versions have that problem in their first releases. And yes, it is a problem with GTK+.
What does that mean?
I had that problem (duplicated characters when writting) in almost all versions since 1.0 'til 1.6, in Fedora, Lubuntu and Xubuntu. The fix in all cases was to wait until an update fixed it (don't ask me who and how, but somebody did). I didn't test 1.8 yet (and I'll don't until version 1.8.1*) so I can't say if I have te same problem right now.
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* In most cases I never use the "just released" version of open source software. I try to wait until the "dot one" version is out. This is just because I'm very bad evaluating and explaining other people bugs (specially in English) . I hope this doesn't sound too selfish, it is not my intention.