optimization with nodes to keep track the most recent nodes does not work with duplicated nodes

[0xDatapunk]

zk-kit.solidity/packages/imt/contracts/InternalQuinaryIMT.sol

Lines 147 to 149 in a4e7136

	if (nodes[0] == self.lastSubtrees[i][0] || nodes[4] == self.lastSubtrees[i][4]) {
	self.lastSubtrees[i][proofPathIndices[i]] = hash;
	}

when the updated node is on a path different from the more recently inserted path, which is recorded in self.lastSubtrees, we still might get (nodes[0] == self.lastSubtrees[i][0] || nodes[4] == self.lastSubtrees[i][4])==True, so self.lastSubtrees may be updated in

zk-kit.solidity/packages/imt/contracts/InternalQuinaryIMT.sol

Lines 147 to 149 in a4e7136

	if (nodes[0] == self.lastSubtrees[i][0] || nodes[4] == self.lastSubtrees[i][4]) {
	self.lastSubtrees[i][proofPathIndices[i]] = hash;
	}

However, _insert would be using wrong values in

zk-kit.solidity/packages/imt/contracts/InternalQuinaryIMT.sol

Line 93 in a4e7136

hash = PoseidonT6.hash(self.lastSubtrees[i]);

This optimization with nodes to keep track the most recent nodes does not work even if the tree does not allow for duplicated values, since update/delete may result in duplicated zero values. Also there is no mechanism to enforce always adding unique leaves, if that is the intention.