Putting new coat of Flood CWF UV cedar tone over old

If using the same stain then you should be good without stripping as long as the old stain is intact and not peeling.

We have used Flood CWF-UV, cedar tone, applied every 3rd year, on a pressure treated pine deck (central Illinois, gets direct sun most of mid-day; deck is 20 years old). This is our year to reapply. However, the product seems to have dried and peeled very badly this time (especially in an area that doesn't get as much sun), and has blackened in some areas and allows mildew (on the part of deck not receiving sun). Do we need to use a stripper instead of just cleaning it? Should we continue using the same Flood sealer product? What cleaning or stripping products should we use?

We would strip it all off and start over with a fresh surface. Try the Restore A Deck products for the stripping and brightening. Stain with TWP 100 Series.