How To Highlight Duplicate Data In Google Sheets 

Highlighting cells that contain duplicates, i.e. the exact same information, can be done in 3 steps in Google Sheets:
1. Step 1, select the area where you wish to check for duplicates.
2. Step 2, click “Format”, “Conditional formatting” and “Custom formula is”.
3. Step 3, write =COUNTIF and specify the range of interest as well as the starting cell for criterion within the parenthesis. Then make the formula true if it counts MORE THAN 1. Also, add dollar signs to the range!
Here I have a bunch of cells that randomise numbers between 1-10 each time I tick or untick this box! The conditional formatting formula looks through all the cells in the range and counts each time it finds the value in the first cell. If the final count is more than 1, it formats this first cell. Then it does the same thing for the second cell, then the third and so on. Each time it checks how many times the value in the current cell occurs and formats the cell in question if its value occurs more than once! The dollar signs prevent the entire range from moving when the cell it checks against changes.