An indication like “f.” means than an identical entry has been found in the same or next page. A tag of “ff.” means identical entries in more consecutive pages.
If you prefer an explicit mention of the limiting pages, adjust the settings in the configuration dialog of Alphabetical Index. You can choose between intervals like 1-3 or exhaustive list like 1, 2, 3. You can also opt for no merging at all of identical entries; identical entries will be listed individually even when there are several on the same page.
You didn’t mention your (OS name nor) LO version. Since wording of the dialog changed a bit in recent version, I let you experiment in your release.