How to prevent rows with duplicated indices / keys to be appended to a data.frame?

I have data in which the combination of two variables ("ManufactererId" and "ProductId") constitute unique keys / identifiers. The data looks like this:

my.data <- data.frame(ManufactererId = c(1, 1, 2, 2),
                      ProductId = c(1, 2, 1, 7),
                      Price = c(12.99, 149.00, 0.99, 3.99))
my.data
#   ManufactererId ProductId  Price
# 1              1         1  12.99
# 2              1         2 149.00
# 3              2         1   0.99
# 4              2         7   3.99

I want to ensure that I cannot accidentally add another row with a pair of ManufactererId - ProductId equal to what is already present in the table (like the unique constraint on a database table).

That is, if I try to add a row with ManufactererId = 2 and ProductId = 7 to my data frame:

my.data <- rbind(my.data, data.frame(ManufactererId = 2, ProductId = 7, Price = 120.00))

...it should fail with an error. How can this be achieved?

Or should I use a different data type?