I have no idea how to delete duplicate songs from my sansa 280e.

You can have a try Leawo Tunes Cleaner. Just follow below steps. If you still have issues or have a massive music library in iTunes or other music folders that requires a more powerful and less time-consuming option, the third-party software like Leawo Tunes Cleaner can help you. It can not only automatically clean up duplicate songs in iTunes, but also automatically delete duplicate songs in other music folders. If you want to automatically remove duplicates in iTunes, the automatic duplicate remover might have the answer for you.
1. After kicking off Leawo Tunes Cleaner, click the "Clean-up Duplicate" entry. The program provides 2 duplicate scanning modes: Quick Scan mode and Deep Scan mode. Select one of them to scan duplicate songs.
2. You could delete duplicates in iTunes music library by directly clicking "Scan iTunes" button or click "Add" button to load music files for duplicates removing.
3. After loading source music files, click the "Scan" button on the upper-right corner to start scanning duplicates. The program would display all duplicates clearly and suggest what to remove. You could decide which duplicates to delete by checking them and then click the "Remove" button to remove duplicate songs.

For more details, check: How to Delete Duplicate Songs

You can try fomatting it.to do so plug it and open my computer.go to the drive of the mp3 and right click it.in the sub menu click format(to foramt it you have to be administrator)(also be sure backup all of your music,videos,pictures,and anything that you want to save.)if you put your songs back on and the problem happens again then format it again and go through the songs and delete the duplicates that you find