Prevents virus stacking (#19539)

code/datums/diseases/_disease.dm

@@ -49,6 +49,9 @@ GLOBAL_LIST_INIT(diseases, subtypesof(/datum/disease))
 
 /datum/disease/proc/infect(mob/living/infectee, make_copy = TRUE)
 	var/datum/disease/D = make_copy ? Copy() : src
+	if(infectee.HasDisease(D))
+		qdel(D)
+		return
 	infectee.addDisease(D)
 	D.affected_mob = infectee
 	GLOB.active_diseases += D

code/modules/reagents/reagents/core.dm

@@ -88,6 +88,8 @@
 				var/datum/disease/D = ID
 				if((D.spread_flags & DISEASE_SPREAD_SPECIAL) || (D.spread_flags & DISEASE_SPREAD_NON_CONTAGIOUS))
 					continue
+				if(M.HasDisease(D))
+					continue
 				D.try_infect(M)
 
 /datum/reagent/blood/affect_touch(mob/living/carbon/M, alien, removed)