Duplicate code is one of the most pungent code smells. A rule that is often used is to re-structure code once it is duplicated in three or more places.
Common duplication problems, and corresponding solutions are:
| 1 | <?php  | 
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| 17 | class SiteConfigSubsites extends DataExtension  | 
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| 18 | { | 
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| 19 | private static $has_one = [  | 
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| 20 | 'Subsite' => Subsite::class, // The subsite that this page belongs to  | 
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| 21 | ];  | 
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| 22 | |||
| 23 | /**  | 
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| 24 | * Update any requests to limit the results to the current site  | 
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| 25 | * @param SQLSelect $query  | 
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| 26 | * @param DataQuery|null $dataQuery  | 
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| 27 | */  | 
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| 28 | public function augmentSQL(SQLSelect $query, DataQuery $dataQuery = null)  | 
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| 58 | |||
| 59 | public function onBeforeWrite()  | 
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| 65 | |||
| 66 | /**  | 
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| 67 | * Return a piece of text to keep DataObject cache keys appropriately specific  | 
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| 68 | */  | 
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| 69 | public function cacheKeyComponent()  | 
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| 73 | |||
| 74 | public function updateCMSFields(FieldList $fields)  | 
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| 78 | }  | 
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| 79 | 
This check marks private properties in classes that are never used. Those properties can be removed.