Code inside Filter executes twice

[SLavrynenko]

NEST/Elasticsearch.Net version:
2.3.1

Elasticsearch version:
"version" : {
"number" : "2.1.1",
"build_hash" : "40e2c53a6b6c2972b3d13846e450e66f4375bd71",
"build_timestamp" : "2015-12-15T13:05:55Z",
"build_snapshot" : false,
"lucene_version" : "5.3.1"
},

Description of the problem including expected versus actual behavior:
I've the following code:

            var response = client.Search<ContactElastic>(
                s => s.Query(
                    q => q.Bool(
                    b => b
                        .Filter(f =>
                        {
                            var filter = f.Term(p => p.ClientId, 1);

                            return filter;
                        }))));

But I see that code inside Filter executes twice, why? Here is a simple code but inside my Filter side I've a sophisticated code that shouldn't to run twice.