Question: PDSolve, Error: duplicated elements (ranking)

Hello! I have some trouble with getting an explicit solution to a PDE: f=f(u,v) in IC^4 (IC=complex numbers) So i have a system of 8 equations, first order, integrable (i.e. solution exists by theory). I'm getting the following, when calling pdsolve(PDE, fcns), where PDE is the set of the PDEs and fcns are the 4 complex functions f[i], i=1,2,3,4: "Error, (in pdsolve/sys) duplicated elements (ranking)" What could it be? Thx, yadaddy.
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