Wash. Rev. Code § 35.18.240
Petitions to reorganize a city or town on the council-manager plan must be signed by registered voters resident therein equal in number to at least twenty percent of the votes cast for all candidates for mayor at the last preceding municipal election. In addition to the signature and residence addresses of the petitioners thereon, a petition must contain an affidavit stating the number of signers thereon at the time the affidavit is made.
Petitions containing the required number of signatures shall be accepted by the city or town clerk as prima facie valid until their invalidity has been proved.
A variation on such petitions between the signatures on the petition and that on the voter's permanent registration caused by the substitution of initials instead of the first or middle names or both shall not invalidate the signature on the petition if the surname and handwriting are the same. Signatures, including the original, of any voter who has signed such petitions two or more times shall be stricken.
RCW 35.18.240