Carissa congesta Wight even (not to say of var. albida Santapau) as well as C. opaca Stapf ex Haines (name given to IIndian specimens of C. spinarum (non L.) are treated as a synonym of C. spinarum. This is followed by both The Plant List and Wikipedia (which generally has reliable information)
This leaves decision between C. spinarum L. (or if we treat C. congesta and C. opaca as distinct) and Carissa carandas.
C. carandas C. spinarum
1. Lateral veins of leaf ca. 8 pairs 1. Lateral veins of leaf 3-5 pairs
2. Leaves 3.5-8 cm long 2. Leaves 2-5 cm long
3. Leaf apex rounded, emarginate 3. Leaf apex acute or acuminate to apiculate
4. Fruit ellipsoid, 15-25 mm long. 4. Fruit subglobose, 5-12 mm
5. Ripe fruit reddish-purple 5. Ripe fruit shining black