kentico_jurajo wrote:
What exactly you mean by the link item?

The action:
Right click a page in the CMS -> New... -> Link to existing document

creates an item in the CMS tree under the page you right clicked. That is the "link item".

That said, I figured out the issue on my own.

It turns out that when you set a custom URL that includes "{-}" in the path, and you then try to add aliases that would then match the custom URL path (IE - custom URL "site/{-}/decorations" == "site/event1/decorations"), you get the collision error.

However, one interesting thing to note, is that if you set the aliases first, then set the custom URL, the error does not occur (this is the path I had taken that did not cause an error).

So, to trigger the error:
1. On a page item (shouldn't really matter, as long as a URL maps to it), set the custom URL to something that includes the "{-}" wildcard. (for example - "site/{-}/decorations")
2. Try to add an alias to the item that would match the custom URL ("site/event1/decorations")

To have an alias that matches the custom URL, without triggering the error:
1. On a page item, leave the default URL and set up an alias ("site/event1/decorations")
2. Change the default URL to a custom URL that includes "{-}" and would match the alias ("site/{-}/decorations")

While the usefulness (or desireability) of having custom URL equal one or more alias is arguable, one of the two above behaviors appears to be not intended.