Is it possible for a creature to have the grappled condition from several sources?
Yes.
If so, does beating the highest atheletics dc to escape break it from all sources of grapple making it pointless for multiple people to grapple something, or does it have to escape from each grapple?
The Escape action (page 470) is defined as "pick one creature or whatever that has you grabbed or whatever; succeed and that target no longer has you grabbed or whatever." So an Escape only gets you free from one person's grappling, and using multiple people is helpful.
If the latter were the case, could you grapple a creature twice (once with each arm) forcing it to make 2 escape checks?
No, because the Escape action says, on a success, "You get free and remove the grabbed, immobilized, and restrained conditions imposed by your chosen target." Your target is a creature, not the creature's arm, so it frees you from all the limbs that creature is grabbing you with.
Edit: See above