As I was adding a field to a content type, I ended up with two Label field text inputs on the form (see screenshot). It took me a while to figure out the steps to reproduce it, but finally hit on them...
1) Select a field type from the Add a new field select list (I selected Taxonomy term)
2) Click to open the select list for "Re-use an existing field". (After I had selected to add a new field, I thought I should check to see if that field already existed.)
3) Scroll the list of options in the select list opened in step 2 above. My mouse pointer happened to be hovering over one of the options, but I didn't actually click on it - just had the list open and one of the options highlighted (because it was where my pointer was).
4) Switch to another program on your computer. I'm on a MacBookPro, and I used Ctrl-tab to switch to another browser.
5) Switch back to the browser with Drupal. You should see two Label fields.