diff --git a/zsh-history-substring-search.zsh b/zsh-history-substring-search.zsh
index fb381ad..57cc3a4 100644
--- a/zsh-history-substring-search.zsh
+++ b/zsh-history-substring-search.zsh
@@ -200,12 +200,19 @@ function _history-substring-search-begin() {
     #
     # Find all occurrences of the search query in the history file.
     #
-    # (k) turns it an array of line numbers.
+    # (k) returns the "keys" (history index numbers) instead of the values
+    # (Oa) reverses the order, because (R) returns results reversed.
     #
-    # (on) seems to remove duplicates, which are default
-    #      options. They can be turned off by (ON).
-    #
-    _history_substring_search_matches=(${(kon)history[(R)(#$HISTORY_SUBSTRING_SEARCH_GLOBBING_FLAGS)*${_history_substring_search_query_escaped}*]})
+    _history_substring_search_matches=(${(kOa)history[(R)(#$HISTORY_SUBSTRING_SEARCH_GLOBBING_FLAGS)*${_history_substring_search_query_escaped}*]})
+
+    # Remove duplicate entries (keeping on the most recent) if HIST_FIND_NO_DUPS is set.
+    if [[ -o HIST_FIND_NO_DUPS ]]; then
+        local -A unique_matches
+        for n in $_history_substring_search_matches; do
+            unique_matches[${history[$n]}]="$n"
+        done
+        _history_substring_search_matches=(${(@n)unique_matches})
+    fi
 
     #
     # Define the range of values that $_history_substring_search_match_index