Here the 1874 RLDS edition added an extra and, perhaps because of the occurrence of yea and near the end of the previous verse: “yea and they did multiply and wax strong in the land” (Alma 50:18). Here in verse 19, the 1908 RLDS edition restored the original reading without the and. For another example of such an intrusive and, see the discussion under Alma 45:11.
Summary: Maintain in Alma 50:19 the instance of yea without any following and.