Can elements in a set be duplicated?

If $A = \{x \mid x \text{ is a letter of the word 'contrast'}\}$

Represent it in a Venn Diagram, and then find the $n(A)$.

Do I need to write the letter 't' twice inside the venn diagram? What should be the answer for $n(A)$?

From wikipedia:

Every element of a set must be unique; no two members may be identical. A multiset is a generalized concept of a set that relaxes this criterion.

No. Sets are collections where repetition and order are ignored.

Therefore in the word contrast we have the letters $\{\sf c,o,n,t,r,a,s\}$ and a simple observation tells us that there are exactly $7$ letters in this set.