The easiest way to check for repeating words is to use Excel's Find and Replace feature. By using this feature, you can also replace the words with something else automatically.

How to Replace Duplicate Letters in Excel. Using the find and replace ability in Microsoft Excel 2010, you can quickly replace duplicate letters, words and cells in a spreadsheet document of any size.

If you need to add a comma after the first word in each cell in Excel using the Replicate or Substitute function, read through this article.