When moving a page A to page B where page B exists, the page Special:MovePage will be reprompt and ask for deletion of page B. When check that and press the button multi times the result is undefinded. If the server is fast, you will move the created redirect and delete the original page ending with a self redirect. When the server is slower (or slave lag?), the page is moved once and than deleted again, resulting in no page existing.

This needs a check to avoid conflicts on performing the same action on the same page at the same time.

A possible way is to include the pageid/revisionid to delete as hidden field and just allow delete of that page.
This will also work when two different user will delete and move the same page at the same time.