Delete some clipboard contents

I have duplicate messages on my clipboard contents...

Assuming you have a clipboard service that supports multiple items on the clipboard,
long-press a textbox to bring up the popup menu,
select the "view clipboard" option (whatever that menu text will be),
select the delete command,
select the item to delete.

Depending on your phone, on it's version of android, and on the clipboard service you have, the instructions will be similar to these above but not necessarily exactly like them.