1. In conventional duopoly theory monopoly profit is greater than duopoly profit p0 > p1 and duopoly fan welfare is greater than monopoly fan welfare f0 < f1. Total welfare its maximized under perfect competition at x*.

2. If the original club retains a l-share of the market then the new club must compensate for damages p0 - lp1. This requires (1-l)p1 > p0 - lp1 and p1 > p0, which contradicts 1.

3. If p0 > p1 then the damage to the existing club p0 - lp1 is always greater than the value of the new club (1-l)p1.

4. Therefore the expansion of a sports league into already occupied markets is always superior in terms of fan and total welfare, but inferior in terms of total league welfare (profit).

5. It also follows that the relocation of dual market clubs is league-superior to conventional league expansion.