During the derivation of the business process execution language (BPEL) from graph-oriented process modeling languages, there exist structures called overlapped patterns, which mix sequential and parallel structures together. The existing duplication approach lacks systematical analysis and formal presentations. In view of this problem, a transformation of UML activity diagrams to BPEL based on WF-nets is proposed. By choosing free choice WF-nets as the foundation of activity diagrams and by utilizing the synthesis rules of live and bounded free choice systems, two types of overlapped patterns are defined. For one type of patter, the duplication method is formally represented, and the transformation equivalence is proved using the concurrent regular expressions of Petri nets. For the other type, the applicable range of the duplication method is described. This method presents a solution in formal representations to the transformation of overlapped patterns in BPEL modeling, and distinguishes the derivation of the block-oriented modeling languages from graph-oriented ones.