• Search for "Käsebrot" -> not found
  
• Create a new Item
• enter Label: "Käsebrot"
• enter Description: "ein Brot mit einem Belag aus Käse"
• Add "Also known as": "Käsestulle"
• try to add a Link to de:Käsebrot
  
• get the following Error-Message: "An error occurred while trying to perform save and because of this, your changes could not be completed."
• click Details: Edit not allowed: Site link [[dewiki:Käsebrot]] already used by item [[Q1795455]].

That is quite frustrating... It took seven steps until I get informed about the Error I made. 

I figured out, that you only check for double Entries when you enter a Label and a Description in the currently selected language in the second Step. That check is also a check for bitwise equality. One different character in the description and you get no warning. If the desctiption and Label equals to an existing item you get the following Error-Message: "Another item (3220891) already has label "Käsebrot" and description "ein Brot mit einem Belag aus Käse" associated with language code en".

I think you should:

• enable the search for Item-Labels
• check the Label when creating a new Item
• add Links to the existing Item in the error-messages

After my tests there are several Items with the Label "Käsebrot", I'm sorry for that.

LB