[SalesForce] Hide/Remove items from lightning app menu

You should be able to avoid underscore.js / lodash if you do something like this:

var uniqueObjs = {};

//create a object with unique leads
con.forEach(function(conItem){
  uniqueObjs[conItem.Id] = conItem;
});
//reinitialise con array
con = [];
var keys = Object.keys(uniqueObjs);

//fill up con again
keys.forEach(function(key){
  con.push(uniqueObjs[key]);
});

If some of your leads are new and don't have ids, you can sometimes make a key out of a combo of fname + lname + email (uppercased or lowercased of course)

In the user's profile, remove the Create access for Case object and system will automatically hide it.

Edit based on comments:

Since, the button cannot be restricted based on profiles, so better to handle that with validation rule so, only specific profiles can create cases and others not.