diff --git a/core/attributedef.class.inc.php b/core/attributedef.class.inc.php
index 731ac9688..9bacfd819 100644
--- a/core/attributedef.class.inc.php
+++ b/core/attributedef.class.inc.php
@@ -295,7 +295,7 @@ abstract class AttributeDefinition
 	 * @param \DBObject $oHostObject
 	 * @param $value Object error if any, null otherwise
 	 *
-	 * @return bool
+	 * @return bool|string true for no errors, false or error message otherwise
 	 */
 	public function CheckValue(DBObject $oHostObject, $value)
 	{
@@ -2273,22 +2273,17 @@ class AttributeLinkedSetIndirect extends AttributeLinkedSet
 		/** @var \AttributeExternalKey $oExtKeyToRemote */
 		$oExtKeyToRemote = MetaModel::GetAttributeDef($this->GetLinkedClass(), $this->GetExtKeyToRemote());
 		$sRemoteClass = $oExtKeyToRemote->GetTargetClass();
-		foreach(MetaModel::ListAttributeDefs($sRemoteClass) as $sRemoteAttCode => $oRemoteAttDef)
-		{
-			if (!$oRemoteAttDef instanceof AttributeLinkedSetIndirect)
-			{
+		foreach(MetaModel::ListAttributeDefs($sRemoteClass) as $sRemoteAttCode => $oRemoteAttDef) {
+			if (!$oRemoteAttDef instanceof AttributeLinkedSetIndirect) {
 				continue;
 			}
-			if ($oRemoteAttDef->GetLinkedClass() != $this->GetLinkedClass())
-			{
+			if ($oRemoteAttDef->GetLinkedClass() != $this->GetLinkedClass()) {
 				continue;
 			}
-			if ($oRemoteAttDef->GetExtKeyToMe() != $this->GetExtKeyToRemote())
-			{
+			if ($oRemoteAttDef->GetExtKeyToMe() != $this->GetExtKeyToRemote()) {
 				continue;
 			}
-			if ($oRemoteAttDef->GetExtKeyToRemote() != $this->GetExtKeyToMe())
-			{
+			if ($oRemoteAttDef->GetExtKeyToRemote() != $this->GetExtKeyToMe()) {
 				continue;
 			}
 			$oRet = $oRemoteAttDef;
diff --git a/core/dbobject.class.php b/core/dbobject.class.php
index 1f0c0877f..a12a0882e 100644
--- a/core/dbobject.class.php
+++ b/core/dbobject.class.php
@@ -1822,13 +1822,13 @@ abstract class DBObject implements iDisplay
 	/**
 	 * Check if the given (or current) value is suitable for the attribute
 	 *
-     * @api
-     * @api-advanced
-     *
-	 * @param string $sAttCode
-	 * @param boolean|string $value true if successful, the error description otherwise
+	 * @api
+	 * @api-advanced
 	 *
-	 * @return bool|string
+	 * @param string $sAttCode
+	 * @param mixed $value if not passed, then will get value using Get method
+	 *
+	 * @return bool|string true if successful, the error description otherwise
 	 * @throws \ArchivedObjectException
 	 * @throws \CoreException
 	 * @throws \OQLException
@@ -2108,23 +2108,75 @@ abstract class DBObject implements iDisplay
 		}
 
 		$aChildClassesWithRuleDisabled = MetaModel::GetChildClassesWithDisabledUniquenessRule($sRuleRootClass, $sUniquenessRuleId);
-		if (!empty($aChildClassesWithRuleDisabled))
-		{
+		if (!empty($aChildClassesWithRuleDisabled)) {
 			$oUniquenessQuery->AddConditionForInOperatorUsingParam('finalclass', $aChildClassesWithRuleDisabled, false);
 		}
 
 		return $oUniquenessQuery;
 	}
 
+	/**
+	 * @param string $sAttCode
+	 * @param mixed $value
+	 *
+	 * @uses m_aCheckWarnings to log to user duplicates found
+	 *
+	 * @since 2.8.0 N°3198 check duplicates if necessary :<br>
+	 *     Before we could only add or remove lnk using the uilinks widget. This widget has a filter based on existing values, and so
+	 *     forbids to add duplicates.<br>
+	 *     Now we can modify existing entries using the extkey widget, and the widget doesn't have (yet !) such
+	 *     a filter. The widget will be updated later on, but as a first step we're checking for duplicates server side. This is a non
+	 *     blocking warning, as it was already possible to add duplicates by other means.
+	 */
+	protected function DoCheckLinkedSetDuplicates($sAttCode, $value)
+	{
+		$oAttDef = MetaModel::GetAttributeDef(get_class($this), $sAttCode);
+
+		if (!($oAttDef instanceof AttributeLinkedSetIndirect)) {
+			return;
+		}
+
+		if ($oAttDef->DuplicatesAllowed()) {
+			return true;
+		}
+
+		/** @var \ormLinkSet $value */
+		$aModifiedLnk = $value->ListModifiedLinks();
+		$sExtKeyToRemote = $oAttDef->GetExtKeyToRemote();
+		$aExistingRemotesId = $value->GetColumnAsArray($sExtKeyToRemote, true);
+		$aExistingRemotesFriendlyName = $value->GetColumnAsArray($sExtKeyToRemote.'_friendlyname', true);
+		$aDuplicatesFields = [];
+		foreach ($aModifiedLnk as $oModifiedLnk) {
+			$iModifiedLnkId = $oModifiedLnk->GetKey();
+			$iModifiedLnkRemoteId = $oModifiedLnk->Get($sExtKeyToRemote);
+			$aExistingRemotesIdToCheck = array_filter($aExistingRemotesId, function ($iLnkKey) use ($iModifiedLnkId) {
+				return ($iLnkKey != $iModifiedLnkId);
+			}, ARRAY_FILTER_USE_KEY);
+
+			if (!in_array($iModifiedLnkRemoteId, $aExistingRemotesIdToCheck, true)) {
+				continue;
+			}
+
+			$iLnkId = $oModifiedLnk->GetKey();
+			$aDuplicatesFields[] = $aExistingRemotesFriendlyName[$iLnkId];
+		}
+
+		if (!empty($aDuplicatesFields)) {
+			$this->m_aCheckWarnings[] = Dict::Format('Core:AttributeLinkedSetDuplicatesFound',
+				$oAttDef->GetLabel(),
+				implode(', ', $aDuplicatesFields));
+		}
+	}
+
 	/**
 	 * Check integrity rules (before inserting or updating the object)
 	 *
-     * **This method is not meant to be called directly, use DBObject::CheckToWrite()!**
+	 * **This method is not meant to be called directly, use DBObject::CheckToWrite()!**
 	 * Errors should be inserted in $m_aCheckIssues and $m_aCheckWarnings arrays
-     *
-     * @overwritable-hook You can extend this method in order to provide your own logic.
-     * @see CheckToWrite()
-     * @see $m_aCheckIssues
+	 *
+	 * @overwritable-hook You can extend this method in order to provide your own logic.
+	 * @see CheckToWrite()
+	 * @see $m_aCheckIssues
      * @see $m_aCheckWarnings
      *
 	 * @throws \ArchivedObjectException
@@ -2140,14 +2192,14 @@ abstract class DBObject implements iDisplay
 
 		$aChanges = $this->ListChanges();
 
-		foreach($aChanges as $sAttCode => $value)
-		{
+		foreach($aChanges as $sAttCode => $value) {
 			$res = $this->CheckValue($sAttCode);
-			if ($res !== true)
-			{
+			if ($res !== true) {
 				// $res contains the error description
 				$this->m_aCheckIssues[] = "Unexpected value for attribute '$sAttCode': $res";
 			}
+
+			$this->DoCheckLinkedSetDuplicates($sAttCode, $value);
 		}
 		if (count($this->m_aCheckIssues) > 0)
 		{
diff --git a/dictionaries/en.dictionary.itop.core.php b/dictionaries/en.dictionary.itop.core.php
index 7c7ef2973..c320f1a4f 100644
--- a/dictionaries/en.dictionary.itop.core.php
+++ b/dictionaries/en.dictionary.itop.core.php
@@ -33,6 +33,8 @@ Dict::Add('EN US', 'English', 'English', array(
 	'Core:AttributeLinkedSet' => 'Array of objects',
 	'Core:AttributeLinkedSet+' => 'Any kind of objects of the same class or subclass',
 
+	'Core:AttributeLinkedSetDuplicatesFound' => 'Duplicates in the \'%1$s\' field : %2$s',
+
 	'Core:AttributeDashboard' => 'Dashboard',
 	'Core:AttributeDashboard+' => '',
 
diff --git a/dictionaries/fr.dictionary.itop.core.php b/dictionaries/fr.dictionary.itop.core.php
index 0bea1b975..54146e648 100644
--- a/dictionaries/fr.dictionary.itop.core.php
+++ b/dictionaries/fr.dictionary.itop.core.php
@@ -31,6 +31,8 @@ Dict::Add('FR FR', 'French', 'Français', array(
 	'Core:AttributeLinkedSet' => 'Objets liés (1-n)',
 	'Core:AttributeLinkedSet+' => 'Liste d\'objets d\'une classe donnée et pointant sur l\'objet courant',
 
+	'Core:AttributeLinkedSetDuplicatesFound' => 'Des doublons sont présents dans le champ \'%1$s\' : %2$s',
+
 	'Core:AttributeDashboard' => 'Tableau de bord',
 	'Core:AttributeDashboard+' => '',