How do you double decibels?

When doubling the field quantity factor by 2 the level increases by 6 dB.

When doubling the energy quantity factor by 2 the level increases by 3 dB.

When doubling the sound pressure value by 2 the level increases by 6 dB.

When doubling the sound intensity value by 2 the level increases by 3 dB.

The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level.

You usually don't double decibels - you double the factor (ratio).

Look at the link "Subjectively perceived loudness (volume), objectively measured sound pressure (voltage), and theoretically calculated sound intensity (acoustic power)".