Deduplicate Sign-ups by Normalised Email

People sign up twice as Ada.Lovelace+news@Gmail.com and adalovelace@gmail.com. Normalise emails to catch duplicates:

• Trim whitespace and lowercase everything.
• For gmail.com and googlemail.com: remove dots from the local part, drop everything from + onwards, and use the domain gmail.com.
• For all other domains: only drop a +tag from the local part.
• Invalid addresses (not exactly one @, or an empty local part or domain) raise ValueError.

Implement normalize_email(email), then find_duplicate_signups(emails), which returns groups (lists of the original strings, in input order) that normalise to the same address, for groups with 2 or more members. Order the groups by first appearance and skip invalid emails.