Section 21. (a) In addition to any other remedies provided by law, any violation of sections two, three, eight, or sections fifteen to twenty, inclusive, which has substantially influenced the action taken by any municipal agency in any particular matter shall be grounds for avoiding, rescinding or cancelling the action on such terms as the interest of the municipality and innocent third persons require.

(b) The state ethics commission, the district attorney for that district, or the city or town or state may bring a civil action against any person who has acted to his economic advantage in violation of said sections two, three, eight and fifteen to twenty, inclusive, and may recover damages for the city or town in the amount of such economic advantage or five hundred dollars, whichever is greater. If there has been no final criminal judgment of conviction or acquittal of the same violation, the state ethics commission, the district attorney or the city or town or state may in the discretion of the court recover additional damages for the city or town in an amount not exceeding twice the amount of the economic recovery or five hundred dollars, whichever is greater, and a judgment for such damages shall bar any criminal prosecution for the same violation.