A catalogue of double-mode high amplitude $\delta$ Scuti stars in the
  Galaxy and their statistical properties
  We present the first catalogue of double-mode and multi-mode high amplitude
$\delta$ Scuti star (HADS) in the Galaxy. The catalogue contains source name,
coordinates, radial modes such as the fundamental (F, period P0),
first-overtone (1O, period P1), second-overtone (2O, period P2), and the
third-overtone (3O, period P3) if available, period ratios, magnitude, and the
relevant literature. Totally, 155 sources were collected until March 2021, in
which 142 HADS with double-mode (132 with F and 1O, and 10 with 1O and 2O), 11
triple-mode, and 2 quadruple-mode. Statistical analysis shows clear features:
P0 lies in a range of 0.05 days to 0.175 days (sample: 132 double-mode HADS
pulsating in F and 1O); P1/P0 lies in a range of 0.761 $-$ 0.787 (sample: 142
with P0 and P1), in which about 90\% in 0.765 $-$ 0.785, which is wider than
previous studies. The Petersen diagram was created with a much larger sample
(144 HADS with P0 and P1) and we find that stars with periods in 0.05 $-$ 0.1
days scatter largely from the updated linear relation (i.e., Eq.1), the reason
of which however needs further investigation. Particularly, we discover that
the ratio P2/P1 (sample: 21 HADS with P1 and P2) equals 0.802$\pm$0.004, which
could be viewed as a possible indicator to identify the modes 1O and 2O for
multi-mode HADS. In addition, several unusual stars are pointed out, which may
need more attention to their pulsations and stellar parameters in the future.
