When you paste notes and dynamics to a place where there are already notes and dynamics, the notes get replaced but the dynamics do not. Dynamics get added (you get for example 2 mf symbols underneath each other).
And while symbols like f/mf/p are printed underneath each other – you see that something’s wrong and can delete one of them --, crescendi are being treated differently: They end up on the very same place in the score (you can’t even see that there are two of them), but they are being placed underneath each other in the parts.
It’s even more of a mess if the dynamics are grouped and linked: when you try to clean up the result by deleting the duplicated symbols, the whole linked stack gets deleted. I have to unlink the stack first…
Please make dynamics replace other dynamics when I paste stuff.