Cells on one tab of a worksheet repeating on another - error

I have a strange problem with a spreadsheet I've set up. It has several tabs with multiple information on each. The sums on each individual tab are linked to the front tab (which I've used as a summary page), but I haven't put any other links in.

For some reason, when I type anything in certain cells on one of the tabs (including the summary but not just that), it appears also on all the others, not necessarily in the same place. If I delete it on the the other tabs where it shouldn't be, it deletes the original on the correct tab, which I want to keep. This has been going on for ages, it annoyed me so much I completely recreated the spreadsheet, from scratch, but it's still happening.

I'm flummoxed! Please can you explain why this is happening?