Carissa

Carissa carandas L. (Images by Balkar Singh & Surajit Koley (Inserted by J.M.Garg))

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Carissa macrocarpa (Eckl.) A.DC. (Cultivated) (Images by Nidhan Singh (Validated by Gurcharan Singh) (Inserted by J.M.Garg))

 
Carissa spinarum L. (Images by J.M.Garg, Neil Soares, Dinesh Valke & Nidhan Singh (Inserted by J.M.Garg) (For more images & complete details, click on the links))

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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