Yes overlap/overwrite. If there are multiple doctors, only 1 will win. Most debuffs overwrite (armour break, monk bleed, …). Only wugushi stack.
Maybe it will be changed.
Cleric deprotected zone test (100 stacks) https://www.youtube.com/watch?v=jSzjGjxQrtw
Fletcher wugushi siaulai https://www.youtube.com/watch?v=aeAibTwGrgs
Fletcher mergen siau https://www.youtube.com/watch?v=r4YtBRvDr6I
Alchemist dig combustion siaulai https://www.youtube.com/watch?v=263LSbGqOGI
vs mobs https://www.youtube.com/watch?v=ZYblJZzEXJ0
Lancer & fencer fantasy library boss https://www.youtube.com/watch?v=4FeYfrH245g
sadhu druid saalus mausoleum mission https://www.youtube.com/watch?v=XhXTVuovXhQ