Do not allow repeat numbers

[Guest]

Max, you already helped me and thanks I now have a list of unique identifier
numbers for patients e.g.
6619960A
6467852A
6467852A
6571565A
0033428A
0033428A
6585892A
6614133A
6593135B
6556949A
2659876B
6470356A
6595987B
6491200A
2645327A
6590005A
6474357A
5706769A
2659876B
2649544A
How can I prevent this number being repeated as I want this to highlight if
I am putting the same patient in twice.As you can see I have repeated the 2nd
no on this sample twice which means I have this patient in twice. Also a lot
of the nos start with the same 2 digits and end with the same letter but are
different numbers or identifiers. HELP Please.I have tried whole number, not
equal to, value=COUNTIF(E:E,E1). It seems to work but could not trust my
understanding of it as there will be thousands of entries on this sheet and I
must not have a duplicate no.

 

[Bearacade]

Start with E1 and put this in the conditional formating

in Condition 1, put in Formula is =COUNTIF(A:A,A1)>1 and change th
format to whatever highlight you want

now drag and fill as far down as you need

Now when you have a repeating number, both of the cells will highligh

 

[Guest]

OK I'm alright with conditional formatting but have I the formula in data
validation correct for preventing entering duplicates. I am working in Column
E