This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
In numerous cases it has been held that full force and credit is to be given to judgments of federal courts obtained m one State or Territory when sought to be enforced in the federal courts in another State or Territory, or the District of Columbia. This is due to the fact that, as the Supreme Court say in Claflin v. Houseman,6 "The United States is not a foreign sovereignty as regards the several States, but is a concurrent, and. within its jurisdiction, a paramount sovereignty. Every citizen of a State is a subject of two distinct sovereignties, having concurrent jurisdiction in the State, concurrent as to place and persons, though distinct as to subject-matter. Legal or equitable rights, acquired under either system of laws, may be enforced in any court of either sovereignty competent to hear and determine such kinds of rights and not restrained by its Constitution in the exercise of such jurisdiction. Thus, a legal or equitable right acquired under state laws may be prosecuted in the state courts, and also, if the parties reside in different States, in the federal courts. So rights, whether legal or equitable, acquired under the laws of the United States, may be prosecuted in the United States courts, or in the state courts, competent to decide rights of the like character and class; subject, however, to this qualification, that where a right arises under a law of the United States, Congress may, if it sees fit, give to the federal courts exclusive jurisdiction."