This article only applies to the old designer. Not sure which designer you have? Check here.
If you're using both the code-editor and the visual designer to design your store, then chances are that you have ran into this annoyance when you try to save your changes from the designer.
What made this happen is that you probably copied and pasted a section from one of your pages and ended up with something like this for example.
Notice the spot that says 'data-barley' on both of those paragraphs. That is what your designer uses to know what to edit and when.
In order for the designer to work they need to be unique. You can simply add a number to the end as shown in the image below to solve the problem.
Notice how the second "index_hiw_heading" was changed to "index_hiw_heading_2". That is all it takes to fix the problem. Once you saved the file from the code editor you should be able to save from the designer again. It's recommended that you refresh the designer once if you have had it opened in a different tab or browser just to make sure the changes you made in the code editor are applied.