better test in register_coercion

src/sage/structure/parent.pyx

@@ -1555,6 +1555,18 @@ cdef class Parent(sage.structure.category_object.CategoryObject):
             Traceback (most recent call last):
             ...
             AssertionError: coercion from Univariate Polynomial Ring in b over Integer Ring to Univariate Polynomial Ring in a over Integer Ring already registered or discovered
+
+        TESTS:
+
+        We check that :trac:`29517` has been fixed::
+
+            sage: A.<x> = ZZ[]
+            sage: B.<y> = ZZ[]
+            sage: B.has_coerce_map_from(A)
+            False
+            sage: B.register_coercion(A.hom([y]))
+            sage: x + y
+            2*y
         """
         if isinstance(mor, map.Map):
             if mor.codomain() is not self:
@@ -1565,7 +1577,8 @@ cdef class Parent(sage.structure.category_object.CategoryObject):
             raise TypeError("coercions must be parents or maps (got %s)" % type(mor))
         D = mor.domain()
 
-        assert not (self._coercions_used and D in self._coerce_from_hash), "coercion from {} to {} already registered or discovered".format(D, self)
+        assert not (self._coercions_used and D in self._coerce_from_hash and
+                    self._coerce_from_hash.get(D) is not None), "coercion from {} to {} already registered or discovered".format(D, self)
         mor._is_coercion = True
         self._coerce_from_list.append(mor)
         self._registered_domains.append(D)