Make sure duplicates are remove from PATH.

Library/Homebrew/PATH.rb

@@ -4,12 +4,12 @@ class PATH
   end
 
   def prepend(*paths)
-    @paths.unshift(*parse(*paths))
+    @paths = parse(*paths, *@paths)
     self
   end
 
   def append(*paths)
-    @paths.concat(parse(*paths))
+    @paths = parse(*@paths, *paths)
     self
   end
 

Library/Homebrew/test/PATH_spec.rb

@@ -13,6 +13,10 @@ describe PATH do
     it "splits an existing PATH" do
       expect(described_class.new("/path1:/path2")).to eq(["/path1", "/path2"])
     end
+
+    it "removes duplicates" do
+      expect(described_class.new("/path1", "/path1")).to eq("/path1")
+    end
   end
 
   describe "#to_ary" do
@@ -31,12 +35,20 @@ describe PATH do
     it "prepends a path to a PATH" do
       expect(described_class.new("/path1").prepend("/path2").to_str).to eq("/path2:/path1")
     end
+
+    it "removes duplicates" do
+      expect(described_class.new("/path1").prepend("/path1").to_str).to eq("/path1")
+    end
   end
 
   describe "#append" do
     it "prepends a path to a PATH" do
       expect(described_class.new("/path1").append("/path2").to_str).to eq("/path1:/path2")
     end
+
+    it "removes duplicates" do
+      expect(described_class.new("/path1").append("/path1").to_str).to eq("/path1")
+    end
   end
 
   describe "#validate" do