The GDPR guidance states that subject access requests can only be refused if they are manifestly unfounded or excessive.
The detailed guidance on what constitutes manifestly unfounded or excessive can be found here
In summary a manifestly unfounded or excessive request is:
Malicious or intends to cause disruption
Repeats previous subject access requests
Overlaps with other subject access requests
The guidance makes no mention about whether or not it would be excessive to request information clearly already known to the subject.
When I say information already known, I mean for example:
Asking a previous employer for information such as:
My salary while I worked there
My start and leaving date
My job title
This would be useful in a situation for example, where a person needs to prove their income for the last several months and the previous employer is refusing to provide an employers letter.
Of particular interest is the following paragraph:
An individual may also want to receive another copy of information they have requested previously. In this situation a controller can charge a reasonable fee for the administrative costs of providing this information again and it is unlikely that this is an excessive request.
This seems to imply that I can ask an organization for another copy of information that I already know. They can charge a fee, but not refuse to comply.