That has been a longstanding issue with Poser, it even cuts the mesh for weightmapped figures (just verified that with V4_WM). The two work-arounds (in addition to just using the original mesh) are to export from POser and then weld the duplicated vertices/seams in another application (e.g. UVMapper or any modeling program) or to load it in Daz Studio instead and export from it. DS does not slice the mesh. The Poser export followed by a weld will change the vertex order, so that won't work for developing morphs (unless you can get a utility to reorder them), but it should work fine for a self-contained animation. All in all, if you need the mesh exported whole, DS is your best bet.