Double line in xtable

I am printing a LaTeX table in R with xtable. I would like to insert a double line (\\[-1.8ex]\hline \hline \\[-1.8ex]) instead of the first line (simple \hlineor \topline).

How could I do it automatically?

Example:

table <- data.frame(a=rep(1,2),b=rep(2,2))
print(xtable(table,type = "latex"),
  hline.after = c(-1, 0, nrow(table)-1,nrow(table)))

Result

\begin{table}[ht]
\centering
\begin{tabular}{rrr}
\hline
& a & b \\ 
\hline
1 & 1.00 & 2.00 \\ 
\hline
2 & 1.00 & 2.00 \\ 
\hline
\end{tabular}
\end{table}

Desiderata:

\begin{table}[ht]
\centering
\begin{tabular}{rrr}
\\[-1.8ex]\hline 
\hline \\[-1.8ex]
& a & b \\ 
\hline
1 & 1.00 & 2.00 \\ 
\hline
2 & 1.00 & 2.00 \\ 
\hline
\end{tabular}
\end{table}

I think your best bet is to use add.to.row, as described in 5.9 here.

In your case it could be something like

library(xtable)
table <- data.frame(a=rep(1,2),b=rep(2,2))
tab <- xtable(table, type="latex")

addtorow <- list(
  pos=list(-1), 
  command=c("\\\\[-1.8ex]\\hline")
)

print(tab, type="latex", add.to.row=addtorow)

producing

Or a bit more elegantly, removing the top line and replacing it with a double one

add <- list(
  pos=list(-1), 
  command=c(
    "\\\\[-2ex]\\hline 
     \\hline \\\\[-2ex]")
)

print(tab, type="latex", add.to.row=add, hline.after=c(0:nrow(table)))

% latex table generated in R 3.5.0 by xtable 1.8-2 package
% Mon Jul 22 18:32:44 2019
\begin{table}[ht]
\centering
\begin{tabular}{rrr}
  \\[-2ex]\hline 
     \hline \\[-2ex] & a & b \\ 
  \hline
1 & 1.00 & 2.00 \\ 
   \hline
2 & 1.00 & 2.00 \\ 
   \hline
\end{tabular}
\end{table}