Answer first
Find the intersection of followers and following.
A mutual means the two accounts currently follow one another in the exported snapshot.
Worked check
GroupSet ruleExample
MutualFollowers ∩ Following440
Not following backFollowing − Followers518
You do not follow backFollowers − Following166
Read the relationship state correctly
The check compares two present-tense sets. “Outgoing only” says where the username appears now; it says nothing by itself about intent or history.
Use mutuals as a relationship view, then confirm time-sensitive cases in the app.
Context before action
Mutual status can change after the export was prepared.
Mutual status can change after the export, so verify accounts that matter.
Choose the next step
Review the relevant group and leave intentional one-way relationships alone.