To assemblies with the same name

Your IDE spit out error like "This solution contains two assemblies with the same name, or the SharePoint server already has an assembly with the specified name."

What to do?

Check to see if the webpart that you're trying to add exists in the "\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\FEATURES" path. if so, delete it.

Uninstall that particular Assembly from c:\windows\assembly folder.

That's it! Try deploying it again and hope this time it will work :)

update: "well, actually just by doing the last part will able you to continue deploy the webpart."

Sharepoint: customizing newform.aspx nightmare

this approach will give you trouble using the file attachment feature and will prompted you with an error message when you try to attach a file in a custom form on the Web site in Windows SharePoint Services 3.0.

“This form was customized not working with attachment"

solution can be found here: Here

or if you did not remove the original listform but just hide it you'll probably get this error:

"attachment problem can't move focus to the control because it is invincible.."

solution can be found here: here
or hotfix

update: you can resolved the issue by installing service pack 2 for the sharepoint 2007, windows sharepoint services and your sharepoint designer 2007.

and...the next problem is when you upload a file the interface doesn't change but when you close it you can find that it is already uploaded.