Have you been using the online file manager to save your files?
Sometimes, a rare glitch which duplicates the content of your website might occur. You can verify this by going to the view-source page of your browser, where after line 82, you can see the DOCTYPE declaration again, happening once again on lines 163, 244 and 325. Deleting the DOCTYPE in line 82 and everything under it should help your website display correctly.
If it was indeed the online file manager that caused this, you may wish to consider using a desktop FTP client to edit your files and save your changes: