TextEdit has both Duplicate & Save As… functionality. Both will do approximately the same thing, ie offer you the opportunity to save a new file with a new name.
The difference is that Save As… will save a new file with a new name - but leave the new file as the one you're currently working on & 'invisibly' close the original. If you don't change the name as you do this, you will of course overwrite your original.
Duplicate will present you with a distinctly different document in front of the other, initially called [old name copy]. You then just Save this one. You now have two documents open, not one.
TextEdit & some other Mac apps will use the normal Save As… key command Cmd ⌘ Shift ⇧ S as Duplicate, not regular Save As… but the other is in the File menu should you specifically need it.
One thing to note additionally if you are not used to Mac is that you can quit & relaunch or reboot at any point during this & both files will re-open exactly as you last left them, even if you didn't save. Only when you close the file will you get the prompt to save.
You must save if you're going to send it to someone else, but so long as you're working on it, it will automatically keep your last edited version [but not the undo queue if you relaunch or reboot].
Lastly, if you are working with what is in effect a Template, then you can protect your original 'master' file so you cannot accidentally overwrite it.
Save your 'master', then go to it in Finder, select the file & hit Cmd ⌘ i or right click > Get Info. In the window that pops up, check the box for Stationery Pad then close the info window.
Then every time you double click your 'master file' to open it, it will automatically open a copy, ready to insert your new information & just Save.