Ordinarily, when sendmail builds (rebuilds) an aliases database, it objects to duplicate keys on the left of the colon:

staff:  bill
staff:  leopold     this is an error

But sometimes—for example, in automating—such duplicates are necessary. In such instances, the -A switch can be used with the AliasFile (A) option (see AliasFile) to cause duplicates to be silently appended:

staff:  bill
staff:  leopold
... silently modified by sendmail to internally become
staff:  bill, leopold

Note that this process is further illustrated in Section 12.4.5.

The -A database switch is useful only with alias files because those are the only files that sendmail rebuilds on its own. Beginning with V8.10, this switch is also useful with the ph type (ph).