Dual sourcing is regularly used by NVidia & AMD to be fair, there's never any issue of quality of parity because they choose the foundry on a per-product basis, the only primary exceptions of dual sourcing being used for the same IC is with Apples SoCs. However other parts like Vega's interposer were still multi-sourced. For Pascal, NVidia used Samsung for their lower end cards & TSMC for the high end. AMD planned to dual source on 7nm but then GloFlo's 7nm vanished, while Samsung doesn't have a first gen 7nm and is instead waiting for EUV to mature, so 7nm being single sourced has been more of an exception than a rule.
Dual sourcing is actually pretty expensive business especially given many foundrys competing nodes arn't directly compatible/portable with each other, some times even with a fully complete design it could take many many months of reconfiguring it to suit a different node, but it's kinda necessary when the stakes & costs are this high, planning to single source is kinda suicide as any little delay could ruin & knock off your whole product line for years, just look at Intel.