Howdy -- please see the following example:

=== expath-pkg.xml
<?xml version="1.0"?>
<package xmlns="http://expath.org/ns/pkg" spec="1.0" name="http://dyfis.net/example/ns-test" abbrev="ns-test" version="0.1.136892+build.20120224.105428">
  <title>Namespace Trouble Test</title>
  <xquery>
    <namespace>http://dyfis.net/example/ns-test/foo</namespace>
    <file>foo.xq</file>
  </xquery>
  <xquery>
    <namespace>http://dyfis.net/example/ns-test/bar</namespace>
    <file>bar.xq</file>
  </xquery>
</package>
=== ns-test/foo.xq
module namespace foo = "http://dyfis.net/example/ns-test/foo";

declare function foo:test() {
  1 + 1
};
=== ns-test/bar.xq
module namespace bar = "http://dyfis.net/example/ns-test/bar";

declare function bar:test() {
  1 + 1
};
=== ...in the basex-gui buffer...
import module namespace foo="http://dyfis.net/example/ns-test/foo";
import module namespace bar="http://dyfis.net/example/ns-test/bar";
1+1
=== ...results in...
Error: [XQST0047] Module is declared twice: 'http://dyfis.net/example/ns-test/bar'.