It's not completely true.
Ter'angreals: not unique
Ter'angreals are used for many purposes, if we group them by their spesific functions we can say they don't have to be unique.
It's also can be said that there could be different shaped but same functional ter'angreals too.
We have some examples which are not unique neither by shape nor their function:
An example of different shape but same function is the a'dam Elayne produces which have no leash between the collar and the bracelet unlike the a'dams produced by Seanchans.
Angreals and Sa'angreals: may be or not unique:
As we know that to create an angreal or a sa'angreal is a long and tiring process. (Creatin an angreal). Their production is not mass but one by one. They have one same function, to draw more of the One Power to those who channel thruogh it.
We can say they are not unique by their function because all have only one and the same function despite the amount of One Power they allow differs. Because their production is not a mass production, we don't expect that the amount they allow should be the same but we can't say it's not possible to make two angreals which have the same limit on the amount.
What about shape? There is no evidence as we know that some angreals or sa'angreals are shaped the same but there is also no counter evidence saying it's impossible.