Each project, NGO, donor, etc. is unique. Since a proposal is directly linked to these things, each proposal should be unique as well. It is perfectly reasonable to use an old proposal as a template or base from which to make edits and changes, however, these changes should be significant. It is not recommended to plagiarize past proposals or send the same proposal out multiple times with just a few words changed. Even when seeking funds from different donors for the same project, each donor will have their own preferences and application procedures that each proposal should follow.