another explicit error for Zygote mistake

src/layers/macro.jl

@@ -113,8 +113,13 @@ function _macro_enzyme(type)
     # One-arg method Duplicated(m::Layer) which allocates & zeros the gradient:
     $EnzymeCore.Duplicated(m::$type) = $EnzymeCore.Duplicated(m, $EnzymeCore.make_zero(m))
 
-    # Not sure we want this, but make Duplicated{<:Layer} callable?
-    (m::$EnzymeCore.Duplicated{<:$type})(xs...) = m.val(xs...)
+    # Make Duplicated{<:Layer} callable:
+    function (m::$EnzymeCore.Duplicated{<:$type})(xs...)
+        Zygote.isderiving() && error("""`Duplicated(flux_model)` is only for use with Enzyme.jl.
+            `Flux.gradient` should detect this, but calling `Zygote.gradient` directly on
+            such a wrapped model is not supported.""")
+        m.val(xs...)
+    end
 
     # Not sure but this does prevent printing of 2nd copy:
     $Optimisers.trainable(m::$EnzymeCore.Duplicated{<:$type}) = (; val = m.val)

test/ext_enzyme/enzyme.jl

@@ -205,4 +205,7 @@ end
     @test_throws ArgumentError Flux.gradient((m,z) -> sum(m.bias)/z, Const(m1.val), Active(3f0))
     # Duplicated
     @test_throws Exception Flux.gradient((m,z) -> sum(m.bias)/z, m1, Duplicated(3f0, 0f0))
+
+    # Using Duplicated within Zygote.gradient is not supported:
+    Zygote.gradient((m,x) -> sum(m(x)), m1, [1,2,3f0])
 end