THE ANOMALY OF THE LEEWARD ISLANDS
The federal colony of the Leeward Islands was formed in 1871 comprising the presidencies of Antigua, Domonica, Montserrat, Nevis, St Christopher and the Virgin Islands, each of which continued to issue its own stamps until replaced by a single federal issue in 1890 - using the new "General Key Plate design". However the 1890s were a period of financial hardship in the West Indies and in 1899 the Virgin Islands was allowed to re-introduce its own stamps, and the other presidencies followed in 1903.
However the federal stamps remained in use and until 1956 the colony was unusual in that for all but a few denominations, two sets of stamps were in use concurrently, including the colonial omnibus issues. In 1953, the Colonial Office correspondence described the situation as unusual but accepted by philatelists, although drewing the line at retaining the federal issues after the federal colony had been dissolved, as was being advocated by the Governor.
(There was a slightly similar situation nearer home - in 1958 when the first British Regional stamps were issued, Monmouthshire was technically part of England. The Welsh regional stamps were issued in the county - listed by Gibbons under "Wales & Monmouthshire", but customers at post offices in the county were given the option of buying the standard British definities if they specifically requested them).