multi: make field names more robust

In case that two pointings are different but have the same field identifier,
the beam creation would have siliently exchanged the field ids.
Now, a unique pointing will get its own field id.

resolve/mosaicing/sky_beamer.py

@@ -75,6 +75,20 @@ def _filter_pointings_generator(observations):
             yield obs
 
 
+def _create_field_name(
+    ii: int,
+    observation: Observation,
+    beam_directions: dict
+) -> str:
+    field_name = observation.direction.name
+    field_name = f'fld_{ii:04}' if field_name == '' else field_name
+
+    if field_name in beam_directions:
+        field_name = f"{field_name}_{ii}"
+
+    return field_name
+
+
 def build_sky_beamer(
     sky_domain: ift.DomainTuple,
     sky_center: SkyCoord,
@@ -135,8 +149,7 @@ def build_sky_beamer(
 
         beam = ift.makeField(sky_domain, beam)
 
-        field_name = oo.direction.name
-        field_name = f'fdl_{ii:03}' if field_name == '' else field_name
+        field_name = _create_field_name(ii, oo, beam_directions)
         beam_directions[field_name] = dict(
             dx=dx, dy=dy, beam=beam, direction=oo.direction)