Okay so I was looking at the Xxcha and saw that they have 2 Diplomats. Does that mean that if they are both on the same planet they could repel ground invasions indefinitely?

Good question. However, I think the basic rules are indicating that this is not possible. From Page 35 of base TI3 rules:

If a Diplomat is present on a planet about to be
invaded by enemy Ground Forces during the
Planetary Invasion step, the Diplomat may delay the
invasion by one round. Simply return the enemy
Ground Force units to their Carrier(s). A planet protected
by a Diplomat may not be protected again by a
Diplomat for the remainder of the round, or for the
next game round.

As you can see, one diplomat protects one planet for one game round. That same planet may not be protected again by "a Diplomat" meaning any diplomat, and that same planet may not be protected during the next game round.

Therefore, it looks like the Diplomat protection is attached to the planet in a way. The diplomat, when present, simply cancels a single invasion. I imagine if you moved the diplomat to another planet, you may be able to protect that new planet from attack as well, even if it was the same diplomat again.

Plus, there's an FAQ that leader abilities do not stack.

Seren said:

+1.

Seren said:

You may be on to something there. It would require a fairly specific series of events, most of which are outside your own control, to happen in just the right order, but I believe by RAW you are correct.

Seren said:

Except Generals, correct.

Thanks you to both of you, that was really bothering me.

Also keep in mind the difference between the terms round and turn in TI3.

One round consists of: One Strategy Phase, one Action Phase (which consists of several turns ) and one Status Phase.