Modeshape allows a REST request such as

{
"foo": "first",
"foo": "second"
}

The above request creates a new node with one property named 'foo' which value is 'second'. This is pretty ambigous:
a) the first property instance is silently ignored,
b) JSON keys are often treated as unordered (ok, one can say that ModeShape API behaves differently, but then it should be documented),
c) JCR 2.0, 3.1.3.1 Same-Name Siblings:

"... Additionally, some repositories may support a node and sibling property having the same name (see 13§5.1.8 Node and Property with Same Name). However, two sibling properties can never have the same name."

In my opinion such request should be rejected.