Node URL path is being altered

I was recently made aware of some behavior involving the use of URL aliases that I don't recall having seen before. When someone creates a new URL alias and points it to an existing node and we then go and look at the existing node, we discover that the URL path for this node has been changed to that of the alias just created. For example:

I have an existing node - node/20 with a URL path (Clean URL) of testpage. I then create a new URL alias of testpagetwo and point it to the existing node/20. At this point everything works as it should. However, when I go to node/20 I find that its URL path has been changed to testpagetwo. Upon further testing, I found that if I create a number of URL aliases, each of which points to the same exisiting node/20, the URL path for the node will reflect that of the URL alias most recently created. Since we have Menu blocks whose display (or not) is dependent on the URL path, the change of the node's URL path can potentially cause a problem.

Shouldn't the URL path entered when creating a new page remain unchanged until changed manually? I seem to remember that is how it was working until recently.

I need to get this resolved or, at the very least, find a workaround. Thanks.