add unique token filter docs #8451

Signed-off-by: Anton Rubin <[email protected]>

_analyzers/token-filters/index.md

@@ -59,7 +59,7 @@ Normalization | `arabic_normalization`: [ArabicNormalizer](https://lucene.apache
 `synonym_graph` | N/A | Supplies a synonym list, including multiword synonyms, for the analysis process.
 `trim` | [TrimFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/miscellaneous/TrimFilter.html) | Trims leading and trailing white space from each token in a stream. 
 `truncate` | [TruncateTokenFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/miscellaneous/TruncateTokenFilter.html) | Truncates tokens whose length exceeds the specified character limit. 
-`unique` | N/A | Ensures each token is unique by removing duplicate tokens from a stream. 
+[`unique`]({{site.url}}{{site.baseurl}}/analyzers/token-filters/unique/) | N/A | Ensures each token is unique by removing duplicate tokens from a stream. 
 `uppercase` | [UpperCaseFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/core/LowerCaseFilter.html) | Converts tokens to uppercase. 
 `word_delimiter` | [WordDelimiterFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/miscellaneous/WordDelimiterFilter.html) | Splits tokens at non-alphanumeric characters and performs normalization based on the specified rules. 
 `word_delimiter_graph` | [WordDelimiterGraphFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/miscellaneous/WordDelimiterGraphFilter.html) | Splits tokens at non-alphanumeric characters and performs normalization based on the specified rules. Assigns multi-position tokens a `positionLength` attribute.

_analyzers/token-filters/unique.md

@@ -0,0 +1,102 @@
+---
+layout: default
+title: Unique
+parent: Token filters
+nav_order: 450
+---
+
+# Unique token filter
+
+The `unique` token filter ensures that only unique tokens are kept during the analysis process, removing duplicate tokens that appear within a single field or text block. 
+
+## Parameters
+
+The `unique` token filter can be configured with `only_on_same_position` parameter. If set to `true`, this token filter will act as `remove_duplicates` token filter and will only remove tokens that are in the same position.
+
+## Example
+
+The following example request creates a new index named `unique_example` and configures an analyzer with `unique` filter:
+
+```json
+PUT /unique_example
+{
+  "settings": {
+    "analysis": {
+      "filter": {
+        "unique_filter": {
+          "type": "unique",
+          "only_on_same_position": false
+        }
+      },
+      "analyzer": {
+        "unique_analyzer": {
+          "type": "custom",
+          "tokenizer": "standard",
+          "filter": [
+            "lowercase",
+            "unique_filter"
+          ]
+        }
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Generated tokens
+
+Use the following request to examine the tokens generated using the analyzer:
+
+```json
+GET /unique_example/_analyze
+{
+  "analyzer": "unique_analyzer",
+  "text": "OpenSearch OpenSearch is powerful powerful and scalable"
+}
+```
+{% include copy-curl.html %}
+
+The response contains the generated tokens:
+
+```json
+{
+  "tokens": [
+    {
+      "token": "opensearch",
+      "start_offset": 0,
+      "end_offset": 10,
+      "type": "<ALPHANUM>",
+      "position": 0
+    },
+    {
+      "token": "is",
+      "start_offset": 22,
+      "end_offset": 24,
+      "type": "<ALPHANUM>",
+      "position": 1
+    },
+    {
+      "token": "powerful",
+      "start_offset": 25,
+      "end_offset": 33,
+      "type": "<ALPHANUM>",
+      "position": 2
+    },
+    {
+      "token": "and",
+      "start_offset": 43,
+      "end_offset": 46,
+      "type": "<ALPHANUM>",
+      "position": 3
+    },
+    {
+      "token": "scalable",
+      "start_offset": 47,
+      "end_offset": 55,
+      "type": "<ALPHANUM>",
+      "position": 4
+    }
+  ]
+}
+```