Multiple copies create confusion
This is true that having backup is one of the most important best practices but it should be maintained in well managed way like you can use tags like name, date and time of the backup, version etc. otherwise if you have multiple copies of the same source code or document, then it will create confusion and it would be difficult to identify latest code or document.
When taking a local backup, if you do not give a proper name or strategy, you end up updating a different file and wonder that why it is not getting reflected in your application isn’t funny. At times older files overrides the latest code and causes trouble. So make a strategy or standard and follow it religiously.
This is strongly recommended to use proper source code version control system. There are many source code version control software freely available like (SCCS, CVS, Subversion etc.) which you can use to store different versions of the software. But while using a source code control system then following the following rule:
Always take source code form the controlling system.
Assign a new version to every change.
Always put source code back into controlling system.