When two portals are built next to each other, they could potentially be exploited for spawining in resources

Going back and forth through one portal and respawning the other portal with arrow (for a little bit, it did not let me though the second one after breaking the overworld side, but shooting an arrow still spawned in more portals. the problem with going through was probably caused by something else though), generating a tower of amethyst, copper and gold ready to mine

Not sure what to suggest, maybe borrowing from nether portal spawning and seeking (if there is a portal close by, teleport to it instead of creating a new one)

I enhanced how portal position is calculated, now portal will search for tile entities instead of blocks and will do it in 3x3 chunk area, so portal will not duplicate itself anymore. Destination portal will be selected randomly, so it will be hard to exploit it for fast travel

Should be fixed in 0.3.0