Macro pops up when certain value is entered

Would a validation rule not suffice ? (rather than VBA)

Using your sample file

Step 1: create a Named Range re: Checklist

Name: _DVCheckList
RefersTo: =Checklist!$A:$A
(you could use a Dynamic Construct pending other requirements)

Step 2: apply a Data Validation rule to Database sheet

Highlight C2:Dn (where n is last row you wish to apply the logic to) and apply a Custom Data Validation rule of:

=1-COUNTIF(_DVChecklist,$C2)

Set Error Alert message as necessary - change Style from "Stop" to "Warning" etc if duplicates are still to be permitted.