Doubling a jam recipe is usually not recommended. The volume makes it harder to achieve the proper consistency. There is no need to add commercial pectin (and the resulting ton of sugar) to a strawberry jam recipe, even though strawberries are low in pectin. A grated tart apple adds sufficient pectin and is indistinguishable in the final result. Others (including me) have had good results with adding a small amount of kiwi. See the good recipe by Cathy Barrow on the New York Times cooking site for strawberry jam with kiwi. As for your runny jam, boiling it up again will not improve it. Use it on ice cream or yogurt. And invest in a good instant read thermometer if you're not using one already.
Agree with Frank Nielander.
Also, is this batch from a reliable recipe you've used before or new to you?
Wondering if the recipe (not the doubling) contributed to the runniness.
Or if this batch of strawberries had more water content than usual.
See for general future info this article from The Guardian about good ratios in jam making
https://www.theguardian.com/science/blog/2013/oct/03/science-magic-jam-making
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Also, is this batch from a reliable recipe you've used before or new to you?
Wondering if the recipe (not the doubling) contributed to the runniness.
Or if this batch of strawberries had more water content than usual.
See for general future info this article from The Guardian about good ratios in jam making
https://www.theguardian.com/science/blog/2013/oct/03/science-magic-jam-making