When you save a file as a Template in Macintosh InDesign CS or CS2, the file is saved as a Mac OS X "Stationery Pad" file. This is a dumb feature of Mac OS X that is poorly implemented. Every time that you open the template, a new file is automatically created. If the template is named "foo.indt", a file named "foo copy.indt" is automatically created in the same folder as the original file.
This is really clunky, because a) it is unlikely that you will want to leave the file named "foo copy.indt"; and b) the filename still carries th .indt filename extension, which indicates that it is still a template file, which is untrue.
The workaround is to save the file as a Template in InDesign, and then close the file. In the Macintosh Finder, select the file copy that is created, and choose File > Get Info. In the Info dialog box, deselect the Stationery Pad option, and select the Locked option.
The next time you open the file, it will open as a "Read-Only" file. When you choose File > Save, you will automatically be prompted for a new file name.
Windows InDesign CS and CS2 doesn't have this limitation, nor does InDesign CS3 on Macintosh or Windows.