Question : How to I stop Excel from copying data across all sheets?

I have an issue where data that I enter is duplicating to all other sheets and in the same cell.  It does not matter which sheet data is entered in as it will duplicate to all the other ones as well.  Any suggestions as to where I should look as I have not found an option to disable this.

Answer : How to I stop Excel from copying data across all sheets?

Are all the tabs currently selected?  Right click a tab and click Ungroup Sheets