Sometimes when you want to add or remove a web part to a page in SharePoint or Office365 pages, if the operation did not complete successfully, you might see a number between brackets next to the web part title ( see next image ), that you simply cannot remove from the web part edit properties panel.
Why did it happen?
The number indicates how many web parts exist on the page ( for example, there are 3 Document Web Parts in the previous image ) even though you might only see one Web Part, the reason is that those duplicated Web Parts was not correctly removed from the page.
You need to remove all duplicated Web Parts including the correct one, then add the Web Part one more time into the page, to do this, follow these 3 steps:
Step 1: Open page in Web Part Maintenance mode
You can open the page in "Web Part Maintenance" mode, by adding ?contents=1 to the URL of your page, as shown on next image.
Step 2: Delete all duplicated Web Parts
In our case, we have 3 duplicated Document Web Parts, select them all and click Delete
Note: the reason we remove all Web Parts ( including the correct one ) because we do not know which one is the correct one.
Step 3: Get back to the page, and add the Web Part back again