Whoever, having been convicted of an offence punishable under Chapter XII or Chapter XVII with imprisonment for a term of 3 years or upwards, is guilty of any offence punishable under either of those Chapters with imprisonment for a term of 3 years or upwards, shall be subject for every such subsequent offence to imprisonment for life, or to double the amount of punishment to which he would otherwise have been liable for the same; but if he is not sentenced to imprisonment for life, he shall not in any case be liable to imprisonment for a term exceeding 15 years.