After r, any consonant (save a spirant before a vowel) is by the grammarians either allowed or required to be doubled (an aspirate, by prefixing the corresponding non-aspirate: 154).

Thus:

अर्क arka, or अर्क्क arkka; कार्य kārya, or कार्य्य kāryya;

a. Some of the authorities include, along with r, also h or l or v, or more than one of them, in this rule.

b. A doubled consonant after r is very common in manuscripts and inscriptions, as also in native text-editions and in the earlier editions prepared by European scholars—in later ones, the duplication is universally omitted.

c. On the other hand, the manuscripts often write a single consonant after r where a double one is etymologically required: thus, kārtikeya, vārtika, for kaārttikeya, vārttika.