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One way is to export the records to a spreadsheet, Sort by "the number" and write a simple calculation in a blank column (lets say "the number" is in col A): IF(A2=A1,"Yes", "No")
or something similar. This will flag your duplicates

Just remember, the results of this calc will change if you re-sort, so you want to either copy and paste the values only into a new column, or just scan down the list manually and mark them.
 
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