English spell-check: "dialoguing"

The English spell-checker does not recognize the form “dialoguing,” wanting to replace it with “dialogging.” I get that from my browser (Opera) spell-checker too. Is this British as opposed to U.S.? I’m confident that U.S. usage is “dialoguing” (following the pattern of “dialogued,” which Opera allows but which Scrivener also wants to change to “dialogged.” I don’t care to get into the merits of the two spellings; I’m just wondering where the -gg- forms come from. They are not recognized by Merriam-Webster, which is the U.S. standard.