(1) It would be too inappropriate of a lower legislative body to legislate on the same subject matter as covered by the higher legislative body and
(2) To subject a citizen to obedience to two laws at the same time on the same subject will be too oppressive.
The basic foundation of the doctrine is that acts of the federal government in a federal system of government bind the states and their agencies as representing separate segments of the federation within the limits of the enumerated powers, in Clyde Engineering Co Ltd v Cowburn (1926) 37 CLR 466, where a federal Act had made provisions on hours of work and a state entered upon the same field to make its own provisions, the High Court of Australia held the state law inoperative. How does the doctrine actually operate in practice? We will consider how the doctrine works in the following aspect:-
Federal Exhaustion, Exclusive & Complete Coverage
It is not always the case that at any time a federal and a state law exist the doctrine applies. There must be an intention in the federal law to completely and exhaustively cover the field and not merely supplementary to cumulative upon state law. How is the complete or exhaustive intention of the federal law shown? In some cases where the federal parliament has power to regulate a particular subject and regulates it in a given manner, and in a given form, it cannot be that state legislatures have a right to interfere, as it were, by way of complement to the legislation of Congress, to prescribe additional regulations. A G Ondo v A G Federation
The ultimate coverage is constitutional coverage. It binds both the federal and state governments. Where the constitution has defined or provided for the exercise of a right in a particular manner, no legislation either by the federal parliament or a state can extent it in a statute short of outright constitutional amendment – Attorney-General of Ogun State v Attorney-General of the Federation  2 NCLR 166, 180-181, or repeat, duplicate or add to subtract from the provision – Attorney-General of Abia State v Attorney-General of the Federation (2002) 9 NSCQLR 670, 785, 788.
Mutually Exclusive Powers
In federal systems such as the Nigeria system where powers are mutually exclusive between the federal and state legislatures, a law passed by the National Assembly on a subject falling within the exclusive jurisdiction of the states is superior to state legislation –A.G Lagos State V A.G federation. Federation apart from this state exclusive zone, there are situations where both National Assembly and State House of Assembly may overlap. In those cases neither legislation is ultra vires if the field of legislation is clear. Where the field is not clear, the National Assembly legislation prevails – A. G Ondo v A. G Federation.
Laws that are beneficial in character fall into a different category. The statutes that are relevant here are those that create duties in emergency or rescue situations. Where the provisions of a federal statute and a state...