Weird how this bug was mentioned 5 years ago and its still happening (in Confluence Server). I just experienced this behavior after creating a custom template and couldn't understand why this was happening.. now that I know it's a bug, I'll stop fretting! Very unusual considering if you try to copy the page created from the custom template, you must change the page name as it will not save if you keep the same name (expected behavior.) But, if you create a new page using the custom template, and that page has the same name as one previously created, it will simply overwrite it without any alerts.
Update: This only happens when I use parameters in the template. When I removed the parameter and step to enter values, the template worked as expected / did not allow creation of a new page with the same title.
User A create a page name "Test". If User B create page from template and name "test", this page "test" can be created and overwrite existed page "test". The creator of page "test" will be "A" and modifier "B" .
example page link " https://A.com.tw/confluence/pages/createpage-entervariables.action?templateId=46006274&spaceKey=SNAA2&title=%E9%9C%80%E6%B1%82%E7%A2%BA%E8%AA%8D%E5%96%AE_&newSpaceKey=SNAA2&fromPageId=46577328"