It seems like sometimes, multiple enchanted mobs can spawn at the exact same location with the exact same enchantments.

I noticed this with zombies, witches, and slimes.

For slimes, since the smaller slimes they split into keep the enchantments, with certain enchant combinations, it even becomes extremely hard if not impossible to defeat the group of slimes. I once had three large slimes with the "Heal allies" enchant. The slimes would all heal each other and it was pretty much impossible to kill them all.

This happened to me in the underground section of Soggy Swamp, and the dungeon in Creeper Woods (where you get access to Creepy Crypt). I played on high levels (IV - VI).

Videos

Soggy Swamp (witches): https://youtu.be/DOerJ7u3LZs
Soggy Swamp (slimes): https://youtu.be/cwH_guQNWkk
Creeper Woods (zombies): https://youtu.be/jCM2POsdD4A