diff --git a/core/src/main/groovy/com/muwire/core/files/FileTree.groovy b/core/src/main/groovy/com/muwire/core/files/FileTree.groovy
index 9f2194f0..4d2f83a3 100644
--- a/core/src/main/groovy/com/muwire/core/files/FileTree.groovy
+++ b/core/src/main/groovy/com/muwire/core/files/FileTree.groovy
@@ -7,7 +7,7 @@ class FileTree {
     private final TreeNode root = new TreeNode()
     private final Map<File, TreeNode> fileToNode = new ConcurrentHashMap<>()
     
-    void add(File file) {
+    synchronized void add(File file) {
         List<File> path = new ArrayList<>()
         path.add(file)
         while (file.getParentFile() != null) {
@@ -31,7 +31,7 @@ class FileTree {
         }
     }
 
-    boolean remove(File file) {
+    synchronized boolean remove(File file) {
         TreeNode node = fileToNode.remove(file)
         if (node == null) {
             return false
diff --git a/gui/griffon-app/views/com/muwire/gui/MainFrameView.groovy b/gui/griffon-app/views/com/muwire/gui/MainFrameView.groovy
index 4bec47e2..cd6f7add 100644
--- a/gui/griffon-app/views/com/muwire/gui/MainFrameView.groovy
+++ b/gui/griffon-app/views/com/muwire/gui/MainFrameView.groovy
@@ -511,7 +511,9 @@ class MainFrameView {
                     public boolean importData(TransferHandler.TransferSupport support) {
                         def files = support.getTransferable().getTransferData(DataFlavor.javaFileListFlavor)
                         files.each {
-                            model.core.eventBus.publish(new FileSharedEvent(file : it))
+                            File canonical = it.getCanonicalFile()
+                            model.core.fileManager.negativeTree.remove(canonical)
+                            model.core.eventBus.publish(new FileSharedEvent(file : canonical))
                         }
                         showUploadsWindow.call()
                         true
@@ -1106,6 +1108,7 @@ class MainFrameView {
         if (rv == JFileChooser.APPROVE_OPTION) {
             chooser.getSelectedFiles().each {
                 File canonical = it.getCanonicalFile()
+                model.core.fileManager.negativeTree.remove(canonical)
                 model.core.eventBus.publish(new FileSharedEvent(file : canonical))
             }
         }