add option to check for preexisting invoices before generating
add option to check for preexisting invoices before generating

puttarajubr/commcare-hq,qedsoftware/commcare-hq,SEL-Columbia/commcare-hq,dimagi/commcare-hq,SEL-Columbia/commcare-hq,puttarajubr/commcare-hq,qedsoftware/commcare-hq,SEL-Columbia/commcare-hq,qedsoftware/commcare-hq,dimagi/commcare-hq,puttarajubr/commcare-hq,qedsoftware/commcare-hq,qedsoftware/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,puttarajubr/commcare-hq