Unity Error--1.error Cs1704:an assembly with the same name ' Unityengine.ui ' has already been

Today in the use of Unity5.5.0, opened yesterday's Unity project has the following error:

1.error Cs1704:an assembly with the same name ' Unityengine.ui ' has the already been. Consider removing one of the references or sign assembly 2./applications/unity 5.5 navmesh/unity S/unityextensions/unity/guisystem/unityengine.ui.dll (Location of the symbol related to previous error) 3./applications /unity 5.5 navmesh/unity Navmesh.app/contents/unityextensions/unity/guisystem/standalone/unityengine.ui.dll ( Location of the symbol related to previous error) 4.error Cs1704:an assembly with the same name ' unityengine.networking ' has already been imported. Consider removing one of the references or sign assembly 5./applications/unity 5.5 navmesh/unity S/unityextensions/unity/networking/unityengine.networking.dll (Location of the symbol related to previous error) 6./app Lications/unity 5.5 navmesh/unity navmesh.app/contents/unityextensions/unity/networking/standalone/ UnityEngine.Networking.dll (Location of the symbol related to PREvious error) 7.error Cs1704:an assembly with the same name ' Unityengine.hololens ' has already the been. Consider removing one of the references or sign assembly 8./applications/unity 5.5 navmesh/unity S/unityextensions/unity/unityhololens/runtime/unityengine.hololens.dll (Location of the symbol related to previous Error) 9./applications/unity 5.5 navmesh/unity navmesh.app/contents/unityextensions/unity/unityhololens/ 
 Runtimeeditor/unityengine.hololens.dll (Location of the symbol related to previous error) 10.

Workaround:

Close the current project; Delete the library folder in the project directory restart the project, Unity said to write to the relevant library, you can solve the problem.