The Dungeon Master's Guide, on page 187, says:
A portable hole is a "similar item" to another portable hole.
Thus, if one puts one portable hole inside another portable hole, they are both destroyed, right?
The rules about combining extradimensional objects seem to be very strict in 5th Edition, but this rule seems to be vaguely worded. I guess the intent is to discourage the use of combinations of extradimensional items, and to punish careless players.
If, on the other hand, there is some reason to stress that each item is only incompatible with items of other designs, then the rule means that each portable hole is compatible with every other portable hole.