How to solve overlapping count clues

A worked logic puzzle example: compare overlapping groups and prove matching identities without guessing.

Two verified records

Exactly one of A, B, C is malevolent. Exactly one of A, B, D is malevolent. Both groups share A and B, so C and D must share an identity. This does not yet establish which identity.

Add one more record

If a third verified record establishes that C is benign, D must also be benign.

Test testimony first

This comparison assumes both records are true. Benign speakers make true claims; malevolent speakers make false claims. Not exactly one does not mean exactly zero: two or more may also be possible.

Compare shared people, establish reliable records, and submit a verdict only when every consistent arrangement agrees.