bought a used 46" unit this spring. in great condition and runs great. the deck drive belt seems to fall off at least once every time i use it. previous owner did not hide the fact that it does happen guess i did not know how often. the pulleys have been changed and belt is tight. any thoughts on how to correct this issue

A model # for the machine might help..
 
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it is a RZ 4621

https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/husqvarna/zero-turn-consumer/rz-4621-967009805-2013-11/mower-deck-cutting-deck 
 
I would check all pulleys and idlers for not only wear but for alignment as well.
 
Check the deck against the parts diagram above for any missing parts. 
 
Worn out moveable spindles / idlers could cause issues (they move on/with a bracket to keep the belt taught) would cause isssues.
A bent deck (thus spindle, etc), would cause issues as well.  Pay attention
to area's of where the belt is excessively flopping/ vibrating about.
 
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