Finding Duplicate text strings with a single column

[Ed P]

I have a list of e-mail addresses. I want to find all the duplicates. I have

sorted the list but the list is extremly long.
I have not found an effective method for doing this. I want the result to
show me where those duplicates occur.
Can anyone help me?
Thank you

Ed

 

[Max]

Give this a try ..

Assuming the email list is in col A, A2 down

Enter a label in B1

Put in B2: =IF(COUNTIF($A$2:A2,A2)>1,"X","")
Copy B2 down

This will mark all duplicates in col A with "x"

Do an autofilter in B1, select "X"

 

[Max]

Alternatively, instead of

Put in B2: =IF(COUNTIF($A$2:A2,A2)>1,"X","")
Copy B2 down

If there's the possibility of whitespaces (invisible)
within the email addresses in col A

Put in B2:
=IF(SUMPRODUCT((TRIM($A$2:A2)=TRIM(A2))*(TRIM(A2)<>""))>1,"X","")
Copy B2 down, as before