Paper with proof that $L=\{ a^n b^n \mid n \geq 0 \} \cup \{ a^n b^{2n} \mid n \geq 0 \}$ is not Deterministic Context Free?
The result is proved in Ginsburg and Greibach, Deterministic context free languages, Inform. Control 9(6), 620–648, 1966, Theorem 4.1 on page 24 (643). However, the proof looks somewhat different.