core-bundle/src/Resources/contao/library/Contao/Database.php
public function isUniqueValue($strTable, $strField, $varValue, $intId=null)
	{
		$strQuery = "SELECT * FROM $strTable WHERE " . static::quoteIdentifier($strField) . "=?";

		if ($intId !== null)
		{
			$strQuery .= " AND id!=?";
		}

		$objUnique = $this->prepare($strQuery)
						  ->limit(1)
						  ->execute($varValue, $intId);

		return $objUnique->numRows ? false : true;
	}
public function isUniqueValue($strTable, $strField, $varValue, $intId=null)
	{
		$strQuery = "SELECT * FROM $strTable WHERE " . static::quoteIdentifier($strField) . "=?";

		if ($intId !== null)
		{
			$strQuery .= " AND id!=?";
		}

		$objUnique = $this->prepare($strQuery)
						  ->limit(1)
						  ->execute($varValue, $intId);

		return $objUnique->numRows ? false : true;
	}
Check whether a field value in the database is unique

@param string  $strTable The table name
@param string  $strField The field name
@param mixed   $varValue The field value
@param integer $intId    The ID of a record to exempt

@return boolean True if the field value is unique