To expand on what Alex already mentioned. There is no such thing as 
double-colon keywords. Double-colon is a reader alias mechanism that let 
the reader resolve them so you can type less.

(ns foo.bar.xyz)

::hello

this is resolved at read-time and identical to actually writing

:foo.bar.xyz/hello

:: without a slash will use the current namespace as the namespace for the 
keyword.

If you have a require for another namespace and use :: with a slash you can 
use it to resolve your require alias.

(ns some.thing
  (:require [foo.bar.xyz :as x]))

::x/hello

again becomes

:foo.bar.xyz/hello

in your actual program. Nothing :: will ever exist at runtime.

So if you want you can always use the fully qualified keyword directly or 
you can let the require resolve them based on your ns require's. That means 
you'll get an error if there is no alias to resolve

(ns user2)

::user1/foo

This is invalid because user2 does not have an alias to user1, instead it 
is an actual full namespace.

:user1/foo

would be fine in this case (and identical to ::foo in user1). Or

(ns user2
  (:require [user1 :as u1]))

::u1/foo

Hope that makes things clearer. 

Cheers,
Thomas

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