diff --git a/app/models/doi.rb b/app/models/doi.rb
index d5a65b3a2..767d644a7 100644
--- a/app/models/doi.rb
+++ b/app/models/doi.rb
@@ -430,6 +430,7 @@ class Doi < ActiveRecord::Base
       # indexes :version_of_count, type: :integer
       indexes :views_over_time, type: :object
       indexes :downloads_over_time, type: :object
+      indexes :citations_over_time, type: :object
       indexes :reference_event_ids, type: :keyword
       indexes :citation_event_ids, type: :keyword
       indexes :reference_events, type: :object
@@ -475,6 +476,7 @@ def as_indexed_json(options={})
       "citation_event_ids" => citation_event_ids,
       "citation_count" => citation_count,
       "citation_events" => citation_events,
+      "citations_over_time" => citations_over_time,
       # "part_ids" => part_ids,
       # "part_count" => part_count,
       # "part_of_ids" => part_of_ids,
@@ -1015,8 +1017,18 @@ def citation_event_ids
     citation_events.pluck(:uuid)
   end
 
+  # remove duplicate citing source dois
   def citation_count
-    citation_events.size
+    citation_events.pluck(:source_doi).uniq.length
+  end
+
+  # remove duplicate citing source dois, 
+  # then show distribution by year
+  def citations_over_time
+    citation_events.pluck(:occurred_at, :source_doi).uniq { |v| v[1] }
+      .group_by { |v| v[0].utc.iso8601[0..3] }
+      .map { |k, v| { "year" => k, "total" => v.length } }
+      .sort_by { |h| h[:year] }
   end
 
   # def part_ids
diff --git a/spec/models/doi_spec.rb b/spec/models/doi_spec.rb
index c000e1e92..33826655e 100644
--- a/spec/models/doi_spec.rb
+++ b/spec/models/doi_spec.rb
@@ -574,7 +574,7 @@
     it "has downloads" do
       expect(doi.download_events.count).to eq(3)
       expect(doi.download_count).to eq(30)
-      expect(doi.downloads_over_time.first).to eq("total"=>10, "yearMonth"=>"2015-06")
+      expect(doi.downloads_over_time.first).to eq("total" => 10, "yearMonth" => "2015-06")
 
       download = doi.download_events.first
       expect(download.target_doi).to eq(doi.uid)
@@ -607,17 +607,22 @@
     let(:client) { create(:client) }
     let(:doi) { create(:doi, client: client, aasm_state: "findable") }
     let(:source_doi) { create(:doi, client: client, aasm_state: "findable") }
-    let!(:citation_events) { create(:event_for_datacite_crossref, subj_id: "https://doi.org/#{doi.doi}", obj_id: "https://doi.org/#{source_doi.doi}", relation_type_id: "is-referenced-by") }
+    let(:source_doi2) { create(:doi, client: client, aasm_state: "findable") }
+    let!(:citation_event) { create(:event_for_datacite_crossref, subj_id: "https://doi.org/#{doi.doi}", obj_id: "https://doi.org/#{source_doi.doi}", relation_type_id: "is-referenced-by", occurred_at: "2015-06-13T16:14:19Z") }
+    let!(:citation_event2) { create(:event_for_datacite_crossref, subj_id: "https://doi.org/#{doi.doi}", obj_id: "https://doi.org/#{source_doi2.doi}", relation_type_id: "is-referenced-by", occurred_at: "2016-06-13T16:14:19Z") }
+    let!(:citation_event3) { create(:event_for_datacite_crossref, subj_id: "https://doi.org/#{doi.doi}", obj_id: "https://doi.org/#{source_doi2.doi}", relation_type_id: "is-cited-by", occurred_at: "2016-06-13T16:14:19Z") }
 
     before do
       Doi.import
       sleep 1
     end
 
+    # removing duplicate dois in citation_count and citations_over_time (different relation_type_id)
     it "has citations" do
-      expect(doi.citation_events.count).to eq(1)
-      expect(doi.citation_event_ids.count).to eq(1)
-      expect(doi.citation_count).to eq(1)
+      expect(doi.citation_events.count).to eq(3)
+      expect(doi.citation_event_ids.count).to eq(3)
+      expect(doi.citation_count).to eq(2)
+      expect(doi.citations_over_time).to eq([{"total"=>1, "year"=>"2015"}, {"total"=>1, "year"=>"2016"}])
 
       citation_event = doi.citation_events.first
       expect(citation_event.source_doi.downcase).to eq(source_doi.uid)