In the folder / directory : MAMP/htdocs/
You can have several web sites:
MAMP/htdocs/
- site1
- site2
- site3
- etc...
Each folder in "MAMP/htdocs/" should consist of a separate entity, or an entire project or web site, that you would then, in turn, define as a site in DW.
Make sure you carefully select the correct folder when defining the site.
Make sure you carefully specify the "path" which will become the URL for testing the site.
I am not sure why you are over-writing new files with old files. Just the opposite should be occurring: You should be overwriting OLD files with NEW files when you save them... (??)
Do yourself a flavor:
1) Read the HELP files in dreamweaver.
HELP
Dreamweaver Help
Tutorials: Working with Dreamweaver Sites
Tutorials: Setting up your site and project files (make sure the "contents" tab is selected)
THEN, read the FAQs on the MAMP Web site.
If you put in a half-hour by reading the help files, you should be able to answer these questions yourself.