Let me give the precise scenario which I witnessed this: a friend of mine was new to the server and tagging along with our party to check out the sights of the server. We did a couple of dungeons and she stayed at the back, because we were all wildly higher level than her.
We go to rest at the nearby temple; she rests once, levels up once to level 3. Two IG hours later, at my heeding, she rests again and levels up to 4.
This implies no XP is lost for "over-levelling" or "level-stacking".