update expr arg check

Author: leo-collins

firedrake/interpolation.py

@@ -12,6 +12,7 @@
 import finat.ufl
 from ufl.algorithms import extract_arguments, extract_coefficients
 from ufl.domain import as_domain, extract_unique_domain
+from ufl.duals import is_dual
 
 from pyop2 import op2
 from pyop2.caching import memory_and_disk_cache
@@ -96,8 +97,8 @@ def __init__(self, expr, V,
         expr = ufl.as_ufl(expr)
         if isinstance(V, functionspaceimpl.WithGeometry):
             # Need to create a Firedrake Argument so that it has a .function_space() method
-            expr_args = extract_arguments(expr)
-            is_adjoint = len(expr_args) and expr_args[0].number() == 0
+            expr_arg_numbers = {arg.number() for arg in extract_arguments(expr) if not is_dual(arg)}
+            is_adjoint = len(expr_arg_numbers) and expr_arg_numbers[0] == 0
             V = Argument(V.dual(), 1 if is_adjoint else 0)
 
         target_shape = V.arguments()[0].function_space().value_shape