My concern is when you restore are the files overwritten ?
Most half-decent backup software should give you the option to either restore to original location or restore to specified location.
Are the files duplicated, which would be unacceptable ?
You would inherently be restoring a copy of the file, so yes there would be duplication.
But the part I didn't make clear was that if you're running a test restore then all you need to do is to pick a small handful of files of various sizes of various ages from various "random" depths on your system and test restore those.
Although your backup software may well have a "verify after backup" function, its only as good as the data being backed up.
I've seen situations where dodgy controller chips start silently randomly corrupting data on disk, the backup process then backs up these corrupted files, the built-in verification passes because the file checksums match (i.e. the backup software blindly backed up the corrupted file). These situations have been on very expensive business-critical systems... so when you're talking about home computers, with cheaper components, then the risk of corruption increases exponentially.
The only way to avoid falling into such a trap is to insert a human performing test restores into the process. Leaving it all to various automated processes will only lead to confirmation bias.