To Reproduce:

1. Build attached (or any other project probably).
2. Build again with Append.
3. You'll get this error:

ArgumentException: Temp/StagingArea/Trampoline/Temp is a directory
System.IO.File.Copy (System.String sourceFileName, System.String destFileName, Boolean overwrite) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.IO/File.cs:109)
UnityEditorInternal.FileMirroring.MirrorFile (System.String from, System.String to, System.Func`3 comparer) (at /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/FileMirroring.cs:24)
UnityEditor.iOS.PostProcessiPhonePlayer.UpdateInstallLocation (UnityEditor.iOS.ProjectPaths paths, ScriptingImplementation backend, BuildOptions options, UnityEditor.iOS.IncludedFileList includedFiles)
UnityEditor.iOS.PostProcessiPhonePlayer.PostProcess (UnityEditor.iOS.iOSBuildPostprocessor pp, BuildTarget target, System.String stagingAreaData, System.String stagingArea, System.String stagingAreaDataManaged, System.String playerPackage, System.String installPath, System.String companyName, System.String productName, BuildOptions options, UnityEditor.RuntimeClassRegistry usedClassRegistry)
UnityEditor.iOS.iOSBuildPostprocessor.PostProcess (BuildPostProcessArgs args)
UnityEditor.PostprocessBuildPlayer.Postprocess (BuildTarget target, System.String installPath, System.String companyName, System.String productName, Int32 width, Int32 height, System.String downloadWebplayerUrl, System.String manualDownloadWebplayerUrl, BuildOptions options, UnityEditor.RuntimeClassRegistry usedClassRegistry) (at /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPipeline/PostprocessBuildPlayer.cs:316)
UnityEditor.HostView:OnGUI()

Also the old project will be corrupted and won't work anymore (missing various il2cpp .h files)