Finding Duplicate Phone Numbers:
you already have Validation (Debra Dalgleish's page),
and you have a helper column (like Chip Pearson's page)
Another option would be to use Conditional Formatting
Select the column or the entire sheet, I think the column probably best.
Duplicate identifications (yellowish first among duplicates, greenish for actual duplicates)
formula 1 is: =IF(COUNTIF($D:$D,$D1)>1,COUNTIF($D$1:$D1,$D1)=1)
formula 2 is: =IF(COUNTIF($D:$D,$D1)>1,COUNTIF($D$1:$D1,$D1)>1)
If you want to treat the first occurrence and duplicate as an exception
then all you need is a formula like this: (note no need to expand the range later on)
Formula 1 is: =COUNTIF($D:$D,$D1)>1
More information on Conditional Formatting
Conditional Formatting
http://www.mvps.org/dmcritchie/excel/condfmt.htm
One advantage of C.F. on a slower computer, touch typing is that
you don't have to wait on validation, and you see the C.F. but
you can continue typing and fix it a few minutes later instead of
interrupting what you are doing.
Another might be you don't need an additional column, so you
are more likely to be able see everything at one time in row.
The methods are all practically the same, as you can tell
from the formulas. Actually the formulas I used don't need
adjusting as you add more rows, but you can see that and
make the change yourself.