diff --git a/taskcluster/gecko_taskgraph/decision.py b/taskcluster/gecko_taskgraph/decision.py
index 84afed528fbd..9fd8a5b5c185 100644
--- a/taskcluster/gecko_taskgraph/decision.py
+++ b/taskcluster/gecko_taskgraph/decision.py
@@ -452,14 +452,17 @@ def set_try_config(parameters, task_config_file):
 def set_decision_indexes(decision_task_id, params, graph_config):
     index_paths = []
     if params["backstop"]:
-        index_paths.append(BACKSTOP_INDEX)
+        # When two Decision tasks run at nearly the same time, it's possible
+        # they both end up being backstops if the second checks the backstop
+        # index before the first inserts it. Insert this index first to reduce
+        # the chances of that happening.
+        index_paths.insert(0, BACKSTOP_INDEX)
 
     subs = params.copy()
     subs["trust-domain"] = graph_config["trust-domain"]
 
-    index_paths = [i.format(**subs) for i in index_paths]
     for index_path in index_paths:
-        insert_index(index_path, decision_task_id, use_proxy=True)
+        insert_index(index_path.format(**subs), decision_task_id, use_proxy=True)
 
 
 def write_artifact(filename, data):
diff --git a/taskcluster/gecko_taskgraph/test/test_util_backstop.py b/taskcluster/gecko_taskgraph/test/test_util_backstop.py
index af9aabd5afdc..fbd7e77cfc00 100644
--- a/taskcluster/gecko_taskgraph/test/test_util_backstop.py
+++ b/taskcluster/gecko_taskgraph/test/test_util_backstop.py
@@ -18,20 +18,22 @@ from gecko_taskgraph.util.backstop import (
     is_backstop,
 )
 
-LAST_BACKSTOP_ID = 0
+LAST_BACKSTOP_PUSHID = 1
 LAST_BACKSTOP_PUSHDATE = mktime(datetime.now().timetuple())
 DEFAULT_RESPONSES = {
     "index": {
         "status": 200,
-        "json": {"taskId": LAST_BACKSTOP_ID},
+        "json": {"taskId": LAST_BACKSTOP_PUSHID},
     },
     "artifact": {
         "status": 200,
         "body": dedent(
             """
             pushdate: {}
+            pushlog_id: {}
         """.format(
-                LAST_BACKSTOP_PUSHDATE
+                LAST_BACKSTOP_PUSHDATE,
+                LAST_BACKSTOP_PUSHID,
             )
         ),
     },
@@ -50,7 +52,8 @@ def params():
         "head_rev": "abcdef",
         "project": "autoland",
         "pushdate": LAST_BACKSTOP_PUSHDATE + 1,
-        "pushlog_id": LAST_BACKSTOP_ID + 1,
+        "pushlog_id": LAST_BACKSTOP_PUSHID + 1,
+        "target_tasks_method": "default",
     }
 
 
@@ -78,8 +81,8 @@ def params():
         pytest.param(
             DEFAULT_RESPONSES,
             {
-                "pushlog_id": LAST_BACKSTOP_ID + 1,
-                "pushdate": LAST_BACKSTOP_PUSHDATE + 1,
+                "pushlog_id": LAST_BACKSTOP_PUSHID + BACKSTOP_PUSH_INTERVAL - 1,
+                "pushdate": LAST_BACKSTOP_PUSHDATE + (BACKSTOP_TIME_INTERVAL * 60) - 1,
             },
             False,
             id="not a backstop",
@@ -87,10 +90,26 @@ def params():
         pytest.param(
             {},
             {
-                "pushlog_id": BACKSTOP_PUSH_INTERVAL,
+                "target_tasks_method": "nothing",
+            },
+            False,
+            id="dontbuild",
+        ),
+        pytest.param(
+            DEFAULT_RESPONSES,
+            {
+                "pushlog_id": LAST_BACKSTOP_PUSHID + BACKSTOP_PUSH_INTERVAL,
             },
             True,
-            id="backstop interval",
+            id="interval",
+        ),
+        pytest.param(
+            DEFAULT_RESPONSES,
+            {
+                "pushlog_id": LAST_BACKSTOP_PUSHID + BACKSTOP_PUSH_INTERVAL + 1,
+            },
+            True,
+            id="greater than interval",
         ),
         pytest.param(
             DEFAULT_RESPONSES,
@@ -138,8 +157,8 @@ def test_is_backstop(responses, params, response_args, extra_params, expected):
                 **{"trust-domain": "gecko", "project": params["project"]}
             )
         ),
-        "artifact": get_artifact_url(LAST_BACKSTOP_ID, "public/parameters.yml"),
-        "status": get_task_url(LAST_BACKSTOP_ID) + "/status",
+        "artifact": get_artifact_url(LAST_BACKSTOP_PUSHID, "public/parameters.yml"),
+        "status": get_task_url(LAST_BACKSTOP_PUSHID) + "/status",
     }
 
     for key in ("index", "status", "artifact"):
diff --git a/taskcluster/gecko_taskgraph/util/backstop.py b/taskcluster/gecko_taskgraph/util/backstop.py
index 26c9a4fb9132..005e76c931a8 100644
--- a/taskcluster/gecko_taskgraph/util/backstop.py
+++ b/taskcluster/gecko_taskgraph/util/backstop.py
@@ -37,22 +37,17 @@ def is_backstop(
         return True
 
     project = params["project"]
-    pushid = int(params["pushlog_id"])
-    pushdate = int(params["pushdate"])
-
     if project in TRY_PROJECTS:
         return False
     if project not in integration_projects:
         return True
 
-    # On every Nth push, want to run all tasks.
-    if pushid % push_interval == 0:
-        return True
-
-    if time_interval <= 0:
+    # This push was explicitly set to run nothing (e.g via DONTBUILD), so
+    # shouldn't be a backstop candidate.
+    if params["target_tasks_method"] == "nothing":
         return False
 
-    # We also want to ensure we run all tasks at least once per N minutes.
+    # Find the last backstop to compute push and time intervals.
     subs = {"trust-domain": trust_domain, "project": project}
     index = BACKSTOP_INDEX.format(**subs)
 
@@ -67,9 +62,7 @@ def is_backstop(
         return True
 
     try:
-        last_pushdate = get_artifact(last_backstop_id, "public/parameters.yml")[
-            "pushdate"
-        ]
+        last_params = get_artifact(last_backstop_id, "public/parameters.yml")
     except HTTPError as e:
         # If the last backstop decision task exists in the index, but
         # parameters.yml isn't available yet, it means the decision task is
@@ -79,6 +72,15 @@ def is_backstop(
             return False
         raise
 
-    if (pushdate - last_pushdate) / 60 >= time_interval:
+    # On every Nth push, want to run all tasks.
+    if params["pushlog_id"] - last_params["pushlog_id"] >= push_interval:
         return True
+
+    if time_interval <= 0:
+        return False
+
+    # We also want to ensure we run all tasks at least once per N minutes.
+    if (params["pushdate"] - last_params["pushdate"]) / 60 >= time_interval:
+        return True
+
     return False