Check reference lists against your sources.
A reference list must correspond exactly to the unique sources cited, in both directions. Sources cited but absent from the list are the serious error, since a reader cannot follow them. Entries listed but never cited are the opposite problem and usually the residue of deleted paragraphs — harmless to a reader but a common source of marks lost for accuracy. Comparing both counts against unique sources catches each.
Mismatch detection
Missing entries = unique sources cited - reference list entries; uncited entries = reference list entries - unique sources cited, each floored at zero
Verification effort
Verification minutes = reference list entries x seconds per entry / 60
Almost always because a paragraph or section was deleted after the reference was added. Reference managers do not remove the bibliography entry when the citation goes.
No — one source cited ten times is still one entry. Only the count of unique sources should match the list length.