When I delete all symbols from RT Quotes window and add new symbols then also when I backfill all RT quotes it starts backfilling old RT QUOTES SYMBOL even if it is not in RT quotes window.
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You don't say that but one can guess that you are using Interactive Brokers.
"Backfill RT quote windows" in this plugin actually backfills subscribed symbols.
Symbols are subscribed NOT only if they are in RT quote window (current page), but also if they are accessed ANY other method (in other RT quote pages, whenever it is chart, analysis, or what ever). You don't have direct control over how plugin manages its symbols. It uses FIFO so it always unsubscribes the OLDEST symbols (least recently accessed) when you hit the data source symbol limit.