sent items in Yahoo mail appear in my Inbox

Why do my sent emails appear in my inbox? I use Yahoo Mail.

I believe you are getting confused by the inbox behanvior when conversations are enabled:

How to turn off conversations in Yahoo Mail | Email Questions

The link above contains the latest directions for turning conversations on & off.

If you prefer individual emails instead of your emails being grouped by topic / conversation, then you'll want to disable the conversation feature.