Okay I don´t know if something like this is already on this forum or not but It kinda creeped me out.
So... I tamed a horse(brown) placed armor and saddle on him and then saw a white one so I tamed that one as well switched the armor from brown to white and then came back to my house. After that I was going to get some xp for enchanting and when I came back there were two white horses with the same armor and the first one was of his lead.
I don´t know what happend here probably some glith but I would like to know more about this.
If "the first one was of his lead" means 'off his lead' (ie the lead was tied to a fence etc but was broken when you returned), this is 'normal' in the sense that it is a long standing bug [ https://bugs.mojang.com/browse/MC-14826 has been known for ~4 years ] that makes leads useless for securing mobs if the player intends to leave the area.
Mobs duplicating (or vanishing) due to glitches (typically when chunks load/unload) has also been previously reported, but does not appear to have been documented as being reproducible.
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.