Short answer - there's no need to strip it, but sometimes it's a good idea.
The two main issues are paint compatibility and loss of detail. For the latter, if you've already added several layers of primer/paint/clear, then adding more on top might start to bury some of the fine detail. Only you can judge based on how thick a coating is on there at present, and what you plan to add on top.
As for compatibility, a really "hot" or aggressive solvent paint over the top of some finishes might give you an issue if you're heavy handed, but that's the same whether it's the 1st, 2nd, or 50th time painting it. Also if there's a concoction of different weathering layers/products/materials built up on it, they need to be borne in mind.
Personally I'd jump straight to respraying it unless I'd loaded it up thick with products the first time around, just treading gently with the application at first.