Da Undertakah reads "Battlecry: gain the deathrattle effects of 3 friendly minions that died this game". He is not a deathrattle minion so if he summons himself he will not retain the deathrattle since summoning a minion will put on the board just a normal copy of it, a simple 8/5 minion.
Now you are saying that this would change with "Copy" instead of gain. I'm sorry to tell you that it doesnt. It only works with faceless because he becomes an exact copy of the minion that it targetted. So when it gets resummoned it summons the other minion. Instead Da Undertakah would yust copy the deathrattle on itself but he will not become another minion. When resummoned it will just summon a basic Da Undertakah: a simple 8/5 Battlecry minion that won't activate its battlecry so it will not gain the deathrattle.
It's all a combination of basic Hs rules used in a particular way that may be misinterpreted.