Consider your bidding plan if your RHO opens 1♦. If you start with a double how do you intend to continue after partner has taken it out?
a) Pass. Too weak for a 1NT overcall. It would be foolish to double because if partner bids 1♠ you may be in a 4-2 fit, An attempted rescue to 1NT would then show 19-20 HCP, compounding your felony.
b) 1NT. Balanced 16-18 HCP and a diamond guard.
c) Double, and then give a single raise in East’s choice of suit. This is consistent with a hand that would open the bidding and then give a jump raise.
d) Double, then rebid 1NT over 1♥ or 1♠ (19-20 HCP) or rebid 2NT over 2♣ (19-21 HCP).
e) 2♥. 12-16 HCP and a good six card suit.
f) Double, and then bid spades at the lowest possible level, showing a hand that is too strong for an immediate 1♠ overcall but lacking the sixth spade for 2♠.
g) Double, then rebid 2NT over 1♥ or 1♠ (21-23 HCP) or rebid 3NT over 2♣ (22 or more HCP).
h) Double, and then jump in hearts, showing a hand too strong for an immediate 2♥ jump overcall.
i) Pass. Despite your strength you have no intelligent bid to make. It is usually wise to pass when so much of your strength is concentrated in the opponents suit.