Comment 4 for bug 1383709

Update, so the ResourceGroup itself isn't causing the increment, it seems the update is triggering a replacement of the ResourceGroup, and the apparent increment is because we create the new ones before deleting the old:

$ heat resource-list overcloud | grep ResourceGroup
| Controller | bc3c047d-f951-4494-9209-7082f13dc335 | OS::Heat::ResourceGroup | CREATE_COMPLETE | 2014-10-23T15:19:17Z |
| Compute | d4f3b141-e8bb-4ae5-8c9d-2ec385ae4e27 | OS::Heat::ResourceGroup | CREATE_COMPLETE | 2014-10-23T15:19:18Z |

<do a stack update, without changing anything>

$ heat resource-list overcloud | grep ResourceGroup
| Compute | 086c398c-f2ef-4f5a-ba5f-7f17071b6bfc | OS::Heat::ResourceGroup | CREATE_IN_PROGRESS | 2014-10-23T16:20:50Z |
| Controller | a6212af5-7a3e-4287-a1a2-14bd93eb41d1 | OS::Heat::ResourceGroup | CREATE_IN_PROGRESS | 2014-10-23T16:20:53Z |

Both the ID's are different, and the nova instances are duplicated:
$ nova list
+--------------------------------------+-------------------------------------------------------+--------+------------+-------------+---------------------+
| ID | Name | Status | Task State | Power State | Networks |
+--------------------------------------+-------------------------------------------------------+--------+------------+-------------+---------------------+
| b10821dd-3edc-446f-b817-c4c2197eccc3 | ov-56swikovh7m-0-6qig4k2hlldz-Controller-4wm3wkupmbpr | BUILD | spawning | NOSTATE | ctlplane=192.0.2.12 |
| bbb2d1a9-56ef-4ab5-91f4-912fc42bb212 | ov-72r43zeuhw-0-7dsvnuujefsw-NovaCompute-xcf4wjohc6iq | BUILD | spawning | NOSTATE | ctlplane=192.0.2.10 |
| 8d58e247-c900-4d56-8086-9968f7ac0d44 | ov-72r43zeuhw-1-o76yax6xvbjc-NovaCompute-dejavowtpml5 | BUILD | spawning | NOSTATE | ctlplane=192.0.2.11 |
| 98c867d0-f2fc-42aa-bad7-1bd291c80c6f | ov-hhgns72yzh-0-it6zqgdcqnvg-NovaCompute-rp5kcn6gqmxo | ACTIVE | - | Running | ctlplane=192.0.2.6 |
| 28f2dcd2-7a28-43ef-99f6-d6b50ebfd989 | ov-hhgns72yzh-1-eqweuh63vcrl-NovaCompute-uszpglj4svss | ACTIVE | - | Running | ctlplane=192.0.2.7 |
| e9f7d3b4-7726-4ced-8e0d-c2cb3e4d59f8 | ov-pghiisxo4ea-0-hk477xeasx5h-Controller-ioo5hg7xvlkh | ACTIVE | - | Running | ctlplane=192.0.2.5 |
+--------------------------------------+-------------------------------------------------------+--------+------------+-------------+---------------------+

Obviously, if the template hasn't changed, we shouldn't be updating anything at all.