MichaelCasper wrote:
I use column "D" for entering invoice numbers.How can I have Excel lock at
all previous enteries to see if I am duplicating an entry on any other row of
column "D"

Hello,

Max gave a good solution. Something else you could try is conditional
formatting:

Place the cursor in D1 and click the D column header to select the
entire column.

Choose Format | Conditional Formatting and fill in the options as follows:

Condition1:
change to Formula is
=COUNTIF(D:D,D1)1

Click the Format button and choose something eye-catching, e.g., yellow
shading with red font.