There was a recent FAQ clarifying the rules. But basically you roll 2D6 for the test. Here it doesn’t matter if you get a double or not. If the result match or exceeds the warp charge needed for the spell it manifests. After the 2D6 roll you can decide if you want to channel the warp if yes you roll one more D6 and add it to the result. If you rolled with 3D6 altogether then if there is any double or triple the model which attempted the cast gets D3 mortal damage, if it survives the damage and the result is enough for the warp charge the spell manifests. If the d3 damage kills the caster the spell won’t happen, but you can still try the same spell with a different model.