While the Details section loops through the data, displaying each value of a field, other sections, such as a Group Header, only displays the first value of a field.  The Group Footer, only displays the last value of a field.

In order to show only the first value for a record, place the desired field(s) in the Group Header section instead of the Details.  If you want the value with the earliest date, you can sort by that particular date field before putting the field(s) in the Group Header.

For example:
If you have a report group by Constituent Sort Key, and you only want to show the first gift a constituent gave, you can sort the data by Gift Date, then put the gift fields in the Group Header for Constituent Sort Key.

To show only the last value for a record, place the desired field(s) in the Group Footer section.