Skip duplicate call signatures

I am seeing `p` twice for os.path.dirname, which seems to come from
Lib/posixpath.py and Lib/ntpath.py, as can be seen with `os.path.join`,
where I still see two with this patch:

```
            (path, *paths)
import os   (a, *p)
os.path.join()
```

pythonx/jedi_vim.py

@@ -475,6 +475,7 @@ def show_call_signatures(signatures=()):
     if int(vim_eval("g:jedi#show_call_signatures")) == 2:
         return cmdline_call_signatures(signatures)
 
+    seen_sigs = []
     for i, signature in enumerate(signatures):
         line, column = signature.bracket_start
         # signatures are listed above each other
@@ -497,6 +498,11 @@ def show_call_signatures(signatures=()):
         except (IndexError, TypeError):
             pass
 
+        # Skip duplicates.
+        if params in seen_sigs:
+            continue
+        seen_sigs.append(params)
+
         # This stuff is reaaaaally a hack! I cannot stress enough, that
         # this is a stupid solution. But there is really no other yet.
         # There is no possibility in VIM to draw on the screen, but there