I agree, you cannot replace with a BJT in a circuit specifically designed where a Darlington transistor is required. I don't mean to steal this thread but the prior response has brought up another issue that I have had recently. I was always taught that 2SAxxxx/2SCxxxx were BJT transistors and 2SBxxxx/2SDxxxx were Darlingtons.
However, I purchased in the past month (12/2018) pairs of 2SB1560's / 2SD2390's reportedly genuine Sanken parts, from four different Sellers on eBay. The first Sellers parts (low priced) all measure with "gains" between 100 to 180; the parts from Seller #2 (higher priced) measured gains of high 90's to 250, with an internal diode from Collector to Emitter. I used three different Chinese transistor testors, which all agreed that these were BJT's not Darlingtons.
I have seen other part numbers of Darlingtons that measure as Darlingtons, with gains upwards of 12,000 to 20,000+ when tested on the same testers, but again, other identical looking parts, which measure gains of under 100. From that I conclude, all these genuine Sanken parts are fakes - BJT parts relabeled as parts that should be Darlingtons, but do not measure as Darlingtons. I also suspect it is very likely the parts I bought from the last two eBay Sellers, which will arrive in two or three weeks, will also be relabeled "fakes" and will measure as BJT's. Other parts that seemingly come from other manufacturers, may similarly be fakes also, this includes parts with a big "K" and parts with a fancy "T".
The conclusion is - buy from First Line Distributors, like Digikey or Arrow, (not an endorsement of any one over any other) when it is important to get the proper part, even if it is triple the cost, there are lots of fakes being touted as "genuine". Second, Test, Test, TEST the parts, and match them where possible, a few of those twenty dollar transistor tester boards with multi-line readouts, do an excellent job of telling you what you have, from Resistors, to Caps, to Diodes, to Transistors, and more! The only thing those testers won't tell you, is the Wattage of Resistors. One place (famous name First Line Distributor) that won't be named, sent me Two Watt Resistors that look like 1/8 Watt from their size, and I am still not believing they are two watts! Even the best of the best screw up sometimes, or get fooled, Buyers Beware!
-Steven