Duplicate language server results when installing a second language plugin

[JPinkney]

Description

When you have a java file and an xml file open in the workspace, then install redhat.vscode-xml followed by redhat.java there is duplicate results from the java language server. It also works in the inverse, if you have both java and an xml file open in the workspace then install redhat.java then redhat.vscode-xml you'll get duplicates from the xml language server.

I'm not using @theia/java or @theia/java-debug in my package.json.

Reproduction Steps

Open a project and create and open a java file and an xml file. Install redhat.vscode-xml and then redhat.java. From inside of the xml file you just created, try doing document symbols. Everything should be working correctly. Then in the java file try doing document symbols and you should see duplicate results.

Video: https://youtu.be/d_NSbRV38BI

OS and Theia version:
Both OS x and Fedora 30 on master

Diagnostics:

I'm getting

root ERROR onWillSave listeners should provide edits, not directly alter the document.

and

rpc-protocol.ts:224 Uncaught (in promise) Error: Assertion failed (document 'file:///home/joshpinkney/Documents/Work/console-java-simple/src/main/java/org/eclipse/che/examples/test.xml' already exists!)
    at Object.ok (:3000/home/joshpinkney/Documents/Test/theia/packages/plugin-ext/lib/common/assert.js:21)
    at EditorsAndDocumentsExtImpl.$acceptEditorsAndDocumentsDelta (:3000/home/joshpinkney/Documents/Test/theia/packages/plugin-ext/lib/plugin/editors-and-documents.js:81)
    at RPCProtocolImpl.doInvokeHandler (:3000/home/joshpinkney/Documents/Test/theia/packages/plugin-ext/lib/common/rpc-protocol.js:207)
    at RPCProtocolImpl.invokeHandler (:3000/home/joshpinkney/Documents/Test/theia/packages/plugin-ext/lib/common/rpc-protocol.js:192)
    at RPCProtocolImpl.receiveRequest (:3000/home/joshpinkney/Documents/Test/theia/packages/plugin-ext/lib/common/rpc-protocol.js:154)
    at RPCProtocolImpl.receiveOneMessage (:3000/home/joshpinkney/Documents/Test/theia/packages/plugin-ext/lib/common/rpc-protocol.js:123)
    at :3000/home/joshpinkney/Documents/Test/theia/packages/plugin-ext/lib/common/rpc-protocol.js:55
    at :3000/home/joshpinkney/Documents/Test/theia/packages/plugin-ext/lib/common/rpc-protocol.js:233
    at :3000/home/joshpinkney/Documents/Test/theia/packages/core/lib/common/event.js:162
    at CallbackList.invoke (:3000/home/joshpinkney/Documents/Test/theia/packages/core/lib/common/event.js:177)

in the console.

[akosyakov]

@JPinkney please try whether it is reproducible against #6159 I found a bug that we spawn many plugin host processes for the same host and it was fixed there.

[JPinkney]

@akosyakov Yeah that fixes it!