> part1<-data.frame(key=c(5,6,7,8,9),x=c("b","d","a","c","b"))
> part1
  key x
1   5 b # key==5,x==b
2   6 d
3   7 a
4   8 c
5   9 b
> part2<-data.frame(key=c(1,2,3,4,5), x=c("c","a","b","d","a"))
> part2
  key x
1   1 c
2   2 a
3   3 b
4   4 d
5   5 a # key==5,x==a

dflist1<-list(part1,part2)
final<-do.call(rbind,dflist1)
final<-final[order(final$key),] #sort by key

> final
   key x
6    1 c
7    2 a
8    3 b
9    4 d
1    5 b #duplicate from part1
10   5 a #duplicate from part2
2    6 d
3    7 a
4    8 c
5    9 b

key x
6    1 c
7    2 a
8    3 b
9    4 d
10   5 a #this is from part2, no more duplicate from part1
2    6 d
3    7 a
4    8 c
5    9 b

## Create many data.frames

set.seed(1)

## Normally, do this in lapply...
part1 <- data.frame(key=1:6,  x=sample(letters, 6))
part2 <- data.frame(key=4:8,  x=sample(letters, 5))
part3 <- data.frame(key=8:12, x=sample(letters, 5))

library(data.table)

## Collect all your "parts"
PARTS.LIST <- lapply(ls(pattern="^part\d+"), function(x) get(x))


DT.part <- rbindlist(PARTS.LIST)

setkey(DT.part, key)
unique(DT.part, by="key")

ORIGINAL          UNIQUE
---------         -----------
> DT.part         > unique(DT.part, by="key")
    key x             key x
 1:   1 l          1:   1 l
 2:   2 v          2:   2 v
 3:   3 k          3:   3 k
 4:   4 q          4:   4 q
 5:   4 i          5:   5 r
 6:   5 r          6:   6 f
 7:   5 w          7:   7 v
 8:   6 f          8:   8 f
 9:   6 o          9:   9 j
10:   7 v         10:  10 d
11:   8 f         11:  11 g
12:   8 l         12:  12 m
13:   9 j
14:  10 d
15:  11 g
16:  12 m