When someone (accidentally) creates an attribute with a name equal to a previously defined attribute, Stardog creates a unique IRI by adding the suffix '_1'.
I noticed, however, that when the duplicate attribute is removed the corresponding IRI isn't always removed from the model. I'm wondering on what basis Stardog decides to remove the IRI? Can you somehow manually remove an IRI in Designer?
There are two ways to disassociate an attribute from a class in Designer. One is deleting the attribute entirely from your project. This can only be done by opening the attribute's drawer and pressing the Delete button (first image). Another way to disassociate an attribute is to Unlink it from a class (second image). Unlinking will not delete the attribute from your project. To fully delete an attribute, you must have the attribute associated with a class and then use the Delete button.
If this does not cover your case, would you mind providing more steps to reproduce? I am sorry if I misunderstand your question!