Name     Symbol       Value
abcd       A            56
de45       C            67
ji98       H            90
k9ug       K            43
phzt       L            98
prex       P            21
kadf       T            32

Symbol     Description
 K            Intel
 P            Diary
 C            Cape
 S            Sheath
 A            Aim

Name     Symbol       Value
abcd       A            56
de45       C            67
k9ug       K            43
prex       P            21

result <- original[original$Symbol %in% duplicate$Symbol,]

merge(original, duplicate, by.x="Symbol", by.y="symbol")
#   Symbol Name Value Description
# 1      A abcd    56         Aim
# 2      C de45    67        Cape
# 3      K k9ug    43       Intel
# 4      P prex    21       Diary

original[original$Symbol %in% duplicate$symbol, ]
#   Name Symbol Value
# 1 abcd      A    56
# 2 de45      C    67
# 4 k9ug      K    43
# 6 prex      P    21

merge(original, duplicate)
original[original$Symbol %in% duplicate$Symbol, ]