diff --git a/src/main/java/net/flintmc/gradle/manifest/tasks/GenerateFlintManifestTask.java b/src/main/java/net/flintmc/gradle/manifest/tasks/GenerateFlintManifestTask.java
index 2c993b3..198ae36 100644
--- a/src/main/java/net/flintmc/gradle/manifest/tasks/GenerateFlintManifestTask.java
+++ b/src/main/java/net/flintmc/gradle/manifest/tasks/GenerateFlintManifestTask.java
@@ -432,30 +432,32 @@ private Set<InstallInstructionModel> buildMavenInstallInstructions(
         remoteMavenRepository = new RemoteMavenRepository(flintGradlePlugin.getHttpClient(), entry.getValue());
       }
 
-      if (!internalRepository.isInstalled(artifact)) {
-        // The artifact is not installed already, install it
-        if (remoteMavenRepository == null) {
-          // Can't download anything in offline mode
-          throw new RuntimeException("Missing artifact " + artifact + " in local repository, " +
-              "but working in offline mode");
-        }
-
-        boolean setupSource = !downloader.hasSource(remoteMavenRepository);
-        if (setupSource) {
-          // The download has the source not set already, add it now
-          downloader.addSource(remoteMavenRepository);
-        }
-
-        try {
-          // Install the artifact including dependencies
-          downloader.installArtifact(artifact, internalRepository);
-        } catch (IOException e) {
-          throw new FlintGradleException("Failed to install maven artifact", e);
-        }
-
-        if (setupSource) {
-          // We added the source, clean up afterwards
-          downloader.removeSource(remoteMavenRepository);
+      synchronized (internalRepository) {
+        if (!internalRepository.isInstalled(artifact)) {
+          // The artifact is not installed already, install it
+          if (remoteMavenRepository == null) {
+            // Can't download anything in offline mode
+            throw new RuntimeException("Missing artifact " + artifact + " in local repository, " +
+                "but working in offline mode");
+          }
+
+          boolean setupSource = !downloader.hasSource(remoteMavenRepository);
+          if (setupSource) {
+            // The download has the source not set already, add it now
+            downloader.addSource(remoteMavenRepository);
+          }
+
+          try {
+            // Install the artifact including dependencies
+            downloader.installArtifact(artifact, internalRepository);
+          } catch (IOException e) {
+            throw new FlintGradleException("Failed to install maven artifact", e);
+          }
+
+          if (setupSource) {
+            // We added the source, clean up afterwards
+            downloader.removeSource(remoteMavenRepository);
+          }
         }
       }
 
diff --git a/src/main/java/net/flintmc/gradle/maven/MavenArtifactDownloader.java b/src/main/java/net/flintmc/gradle/maven/MavenArtifactDownloader.java
index db47c46..e340ce6 100644
--- a/src/main/java/net/flintmc/gradle/maven/MavenArtifactDownloader.java
+++ b/src/main/java/net/flintmc/gradle/maven/MavenArtifactDownloader.java
@@ -119,10 +119,12 @@ public synchronized void installAll(MavenArtifact artifact, SimpleMavenRepositor
     }
 
     if (installIfNotExists) {
-      // If the artifact is not installed locally, try to install it
-      if (!target.isInstalled(artifact) && !installArtifact(artifact, target) && artifactPom == null) {
-        // The artifact failed to install and there was also no POM for it
-        throw new MavenResolveException("Could not resolve " + artifact);
+      synchronized (target) {
+        // If the artifact is not installed locally, try to install it
+        if (!target.isInstalled(artifact) && !installArtifact(artifact, target) && artifactPom == null) {
+          // The artifact failed to install and there was also no POM for it
+          throw new MavenResolveException("Could not resolve " + artifact);
+        }
       }
     }
   }
@@ -185,10 +187,12 @@ private void installAll(MavenPom pom, SimpleMavenRepository target) throws IOExc
           }
         }
 
-        // Try to install the dependency locally
-        if (!target.isInstalled(dependency) && !installArtifact(dependency, target) && dependencyPom == null) {
-          // The dependency had no artifact and also no POM
-          throw new MavenResolveException("Could not resolve " + dependency);
+        synchronized (target) {
+          // Try to install the dependency locally
+          if (!target.isInstalled(dependency) && !installArtifact(dependency, target) && dependencyPom == null) {
+            // The dependency had no artifact and also no POM
+            throw new MavenResolveException("Could not resolve " + dependency);
+          }
         }
       }
     }