Suppose we have a class C with MRO (C, B, A, object).  C virtual-inherits an ABC V, while B virtual-inherits an unrelated ABC W:

       object
      /   |  \
      A   W  |
      | .`  /
      B`  V
      | .`
      C`

Recalling that per PEP 443 singledispatch prefers concrete bases to virtual bases, we would expect the following composed MRO:

    C, B, V, A, W, object

However what actually happens is the composed MRO depends on the order of the haystack; if W is processed first the result is correct, but if V is processed first then (because V does not subclass W) W is inserted in the MRO *before* V:

    C, B, A, object
    C, B, V, A, object
    C, B, W, V, A, object

This results in ambiguity between V and W.

Suggested fix is a slight change to the MRO composition algorithm, considering whether the items already placed in the MRO are concrete base classes.