Of course this is all theoretically suspect as it is impossible (according to the rules of quantum physics) to create an exact copy of an existing state. Thus it would be impossible for your mystery portal material to form an exact parallel universe.
But putting that argument aside, your portal appears to act like a mathematical operator with an inverse. To illustrate: if you construct a portal $A$ then going through it once takes you from your universe $x$ to a parallel (initially 'identical' but now divergent) universe $y$. Passing through it again does the inverse operation - taking you from universe $y$ back to universe $x$. That is fine as lots of operators are their own inverses (for example multiplication by -1).
So mathematically $y = Ax$ and $x = A^{-1}y$ . 28
This is consistent as
$A^{-1} y = A^{-1}(Ax) = (A^{-1}A)x = Ix = x$
But now consider adding a second portal $B$. To simplify the argument imagine constructing both of them simultaneously in universe $x$, so now you have a choice of going through portal $A$ to universe $y$, or through portal $B$ to universe $z$. Note that as you constructed them simultaneously, both $y$ and $z$ will have 'copies' of $A$ and $B$.
Here is where it gets interesting. Going back and forth through portal $A$ just takes you back and forth between $x$ and $y$.
Similarly going back and forth through portal $B$ takes you back and forth between $x$ and $z$.
If you go through $A$, then through $B$, then back through $B$ and back through $A$, you will find yourself in $B^{-1}BA^{-1}Ax = (B^{-1}B)(A^{-1}A)x = IIx = x$, so you end up back home again.
But if instead you go through $A$, then through $B$ and then 'back' through $A$ and then 'back' through $B$ you will be in $B^{-1}A^{-1}BAx$ and if $A$ and $B$ are non-commutative operators then $x = B^{-1}BA^{-1}Ax$ $\ne$ $B^{-1}A^{-1}BAx = x'$
If you repeat the path again and again you will get from $x'$ to $x''$ to $x'''$ etc. so you will cascade through an infinite hall-of-mirrors of alternate universes.
If you want things to be that way then fine, but if you don't want this to be a problem, all you need to do is define that in your universe 'portal-operators' are commutative operators (via some mysterious property of the portal material itself), which would dismiss all recursive loops as then $x = B^{-1}BA^{-1}Ax$ $=$ $B^{-1}A^{-1}BAx = x$ by definition. 945