Fucking spam filter

GMail's spam filter seems to have gotten worse lately. Lots of "you have won!!! blaha blaha" is getting through and at the same time many non-spam mails to mailing lists are classified as spam. Why should it be so hard for GMail? They could just calculate a hash or check sum for each mail on their servers. Then if more than say 10,000 GMail users receive the exact same mail it must be spam. Problem solved, no?