emails won't delete

What's going on with my Mac? I can't delete my old emails. I've tried deleting them one by one. I've tried edit and select all and then gone to mailbox and directed them to be erased and sent to trash..........nothing. Can't figure it out. Also.....a strange thing in that if I send one message it may appear in my mail 6 or more times.
Can anybody help me try to correect this?

Hi Slooose - welcome to the forum!

You might want to provide some more information: 1) Mac model/year; 2) OS X used; 3) Whether you're using Mail (version?); 4) Accounts in Mail; and 5) Whether you've made changes in these accounts, have a new ISP, or whether the mail servers (e.g. Gmail, Yahoo, etc.) used have changed? May help others provide you more specific suggestions - good luck!