Any chance you've already got something installed from the same vendor? This, perhaps?:
https://www.concrete5.org/marketplace/addons/mega-menu1...

Yes I have. I was trying to install another addon but cannot get passed this error.

My guess is that the vendor copied-and-pasted his/her packaging code, and forgot to change the class name to something unique for the new package. Your best bet would be to contact him/her via the support link on the add-on's page. He/she should be grateful to you for picking up this problem.

If that isn't possible, it would be feasible to change the class name throughout the package yourself, but there could be side-effects.

Thanks, I will contact them.

I found the problem. I had renamed the Megamenu folder by adding xxx to it then copied the new Megamenu folder into the packages folder. I have now removed the renamed folder and everything is working correctly now.
Many thanks for responding quickly.
Phil

Ah, that was going to be my next guess. ;)

Glad it's working.