concat.R

library(dplyr)
library(magrittr)

set.seed(1237)
set_df <- function() {
  data.frame(MRN = sample(1:5, 10, replace = TRUE), source = sample(letters[1:20], 10), value = sample(1:10, 10))
}

df1 <- set_df()
df2 <- set_df()

bind_rows(df1, df2) %>% 
  group_by(MRN) %>% 
  summarise(total_value = sum(value),
            sources = paste(source, collapse = "_"),
            values = paste(value, collapse = "_"))

#Source: local data frame [5 x 4]
#
#MRN total_value       sources         values
#(int)       (int)         (chr)          (chr)
#1     1          33   d_g_i_t_b_s    8_1_9_9_4_2
#2     2          16       c_e_h_c        5_2_1_8
#3     3           5             k              5
#4     4          14           t_l           4_10
#5     5          42 q_l_m_o_p_j_e 7_10_3_6_6_7_3