diff --git a/homu/main.py b/homu/main.py
index 5bb00fe7..76dca037 100644
--- a/homu/main.py
+++ b/homu/main.py
@@ -220,7 +220,7 @@ def sha_cmp(short, full):
 def sha_or_blank(sha):
     return sha if re.match(r'^[0-9a-f]+$', sha) else ''
 
-def parse_commands(body, username, repo_cfg, state, my_username, db, *, realtime=False, sha=''):
+def parse_commands(body, username, repo_cfg, state, my_username, db, *, realtime=False, sha='', pending=False):
     try_only = False
     if username not in repo_cfg['reviewers'] and username != my_username:
         if username == state.delegate:
@@ -360,7 +360,11 @@ def parse_commands(body, username, repo_cfg, state, my_username, db, *, realtime
             found = False
 
         if found:
+            if realtime and pending:
+                state.add_comment('Another test is already running on this repository. This test will begin once the other test is finishied.')
             state_changed = True
+            if state.status == 'pending' and realtime:
+                state.add_comment('Please wait until the current test finishes before starting the next test.')
 
             words[i] = ''
 
@@ -612,12 +616,18 @@ def start_build_or_rebuild(state, repo_cfgs, *args):
 def process_queue(states, repos, repo_cfgs, logger, buildbot_slots, db, git_cfg):
     for repo_label, repo in repos.items():
         repo_states = sorted(states[repo_label].values())
+         
+        # find out whether a test is running on this repository
+        pending = False
+        for s in repo_states:
+            if s.status == 'pending':
+                pending = True
 
         for state in repo_states:
             if state.status == 'pending' and not state.try_:
                 break
 
-            elif state.status == '' and state.approved_by:
+            elif state.status == '' and state.approved_by and not pending:
                 if start_build_or_rebuild(state, repo_cfgs, buildbot_slots, logger, db, git_cfg):
                     return
 
@@ -630,7 +640,7 @@ def process_queue(states, repos, repo_cfgs, logger, buildbot_slots, db, git_cfg)
                     return
 
         for state in repo_states:
-            if state.status == '' and state.try_:
+            if state.status == '' and state.try_ and not pending:
                 if start_build(state, repo_cfgs, buildbot_slots, logger, db, git_cfg):
                     return
 
diff --git a/homu/server.py b/homu/server.py
index 4e768cb7..c7679bd3 100644
--- a/homu/server.py
+++ b/homu/server.py
@@ -218,6 +218,12 @@ def github():
 
             state = g.states[repo_label][pull_num]
 
+            pending = False
+            if state.status != 'pending':
+                for state in g.states[repo_label].values():
+                    if state.status == 'pending':
+                        pending = True
+
             if parse_commands(
                 body,
                 username,
@@ -227,6 +233,7 @@ def github():
                 g.db,
                 realtime=True,
                 sha=original_commit_id,
+                pending=pending,
             ):
                 state.save()
 
@@ -334,6 +341,12 @@ def github():
             state.title = info['issue']['title']
             state.body = info['issue']['body']
 
+            pending = False
+            if state.status != 'pending':
+                for state in g.states[repo_label].values():
+                    if state.status == 'pending':
+                        pending = True
+
             if parse_commands(
                 body,
                 username,
@@ -342,6 +355,7 @@ def github():
                 g.my_username,
                 g.db,
                 realtime=True,
+                pending=pending,
             ):
                 state.save()