I am a novice with Google sheet, please pardon my ignorance. I inherited a data sheet that I am trying to improve. It's a massive data that continues to grow each day.
The daily task: I receive a list of phone numbers that I register online. We use a Google sheet to track all the numbers we are registering for each client of ours. In the sheet I have a column with the phone numbers of each client. These are separated by line breaks within each cell. 
How am I able to highlight or know which individual numbers within the cells are duplicated? Is that even possible with a multiline cell?
Any advice is helpful. Thank you!