[antlir] Avoid rendering the same shape target twice

Summary: See upwards in the stack. This trips up builds where the same shape is rendered twice from a single TARGETS file.

Test Plan: Built in this stack.

Reviewed By: cooperlees

Differential Revision: D47555763

fbshipit-source-id: c35b476825694e229b79eb7927e35244cd76e5ec

antlir/bzl/shape.bzl

@@ -696,6 +696,8 @@ def _render_template(name, instance, template, visibility = None):  # pragma: no
     Warning: this will fail to serialize any shape type that contains a
     reference to a target location, as that cannot be safely cached by buck.
     """
+    if native.rule_exists(name + "--data.json"):
+        return normalize_target(":" + name)
     _json_file(name + "--data.json", instance)
 
     buck_genrule(