Add blank rows in between existing rows

Question

I have a dataset of 240 cases, in which I want to create a blank row after each existing row. Leaving me with 480 rows, of which half is filled and the other half is empty (which I then want to fill with some data myself).

Example of data

  id groep_MNC zkhs fbeh    pgebdat    p_age pgesl
1  3         1    1    1 1955-12-01 42.50000     1
2  5         1    1    1 1943-04-09 55.16667     1
3  7         1    1    1 1958-04-10 40.25000     1
4 10         1    1    1 1958-04-17 40.25000     1
5 12         1    1    2 1947-11-01 50.66667     1
6 14         1    1    2 1952-02-02 46.41667     1

Ideally, 'id' should be copied, thus looking like this:

    id groep_MNC zkhs fbeh    pgebdat    p_age pgesl
1    3         1    1    1 1955-12-01 42.50000     1
2    3        NA   NA   NA         NA       NA    NA
3    5         1    1    1 1943-04-09 55.16667     1
4    5        NA   NA   NA         NA       NA    NA
5    7         1    1    1 1958-04-10 40.25000     1
6    7        NA   NA   NA         NA       NA    NA
7   10         1    1    1 1958-04-17 40.25000     1
8   10        NA   NA   NA         NA       NA    NA
9   12         1    1    2 1947-11-01 50.66667     1
10  12        NA   NA   NA         NA       NA    NA
11  14         1    1    2 1952-02-02 46.41667     1
12  14        NA   NA   NA         NA       NA    NA

I've tried copying all the rows with this code:

mydf_long <- mydf[rep(1:nrow(mydf), each = 2),]

But as you can see, that is not even close to what I want to end up with.

Edit: Thanks for the edits and comments. I need to transform my original data to a format that is suitable for multilevel analyses. However, the data is still quite messy so other approaches that initially worked on a small subset of my data, didn't work on my full set. For more information about the background, see my other questions:

Reshape/gather function to create dataset ready for multilevel analysis

Tidy up and reshape messy dataset (reshape/gather/unite function)?

R - replace values by row given some statement in if loop with another value in same df

Since I have relative 'few' partner variables, I now want to create blank lines, and fill them in with the partner data.

Answer 1

We can duplicate each row and then set the row with even row numbers to be NA.

dt2 <- dt[rep(1:nrow(dt), each = 2), ]
dt2[1:nrow(dt2) %% 2 == 0, ] <- NA

head(dt2)
    id groep_MNC zkhs fbeh    pgebdat    p_age pgesl
1    3         1    1    1 1955-12-01 42.50000     1
1.1 NA        NA   NA   NA       <NA>       NA    NA
2    5         1    1    1 1943-04-09 55.16667     1
2.1 NA        NA   NA   NA       <NA>       NA    NA
3    7         1    1    1 1958-04-10 40.25000     1
3.1 NA        NA   NA   NA       <NA>       NA    NA

DATA

dt <- read.table(text = "  id groep_MNC zkhs fbeh    pgebdat    p_age pgesl
1  3         1    1    1 1955-12-01 42.50000     1
2  5         1    1    1 1943-04-09 55.16667     1
3  7         1    1    1 1958-04-10 40.25000     1
4 10         1    1    1 1958-04-17 40.25000     1
5 12         1    1    2 1947-11-01 50.66667     1
6 14         1    1    2 1952-02-02 46.41667     1",
                 header = TRUE, stringsAsFactors = FALSE)