While not exactly the same problem that you solved, there has been previous results considering nonuniqueness of solutions (with zero initial data) for power-law type semilinear term. Interestingly, [contrary to what you wrote](http://mathoverflow.net/questions/137384/spatially-inhomogeneous-solutions-to-parabolic-pde-with-homogeneous-initial-data#comment354552_137384), Lipschitz may not be enough (depending on the function spaces in consideration) for uniqueness. 

Some relevant papers: In the case where the nonlinearity is Lipschitz and the function spaces used are $L^p$ type spaces, we have

- Haraux and Weissler. "Nonuniqueness for a semilinear initial value problem". http://www.ams.org/mathscinet-getitem?mr=648169
- Ni and Sacks. "Singular behavior in nonlinear parabolic equations". http://www.ams.org/mathscinet-getitem?mr=768731
- Baras. "Non-unicité des solutions d'une équation d'évolution non-linéaire". http://www.numdam.org/item?id=AFST_1983_5_5_3-4_287_0

In the case where the nonlinearity is _not_ Lipschitz, we have 

- Fujita and Watanabe, "On the uniqueness and non-uniqueness of solutions of initial value problems for some quasi-linear parabolic equations". http://www.ams.org/mathscinet-getitem?mr=234129

This should be enough to get you started with the literature search on MathSciNet.