Is it safe to breast feed four month old while pregnant again?

Patient: Is it okay to breast feed a 3 month old baby while pregnant?my wife gave birth almost 4 months ago and she is pregnant again, maybe 1 month pregnant.1 She took 2 preg. test last Dcc 18. the second line is light color.2 She took another pregnancy test Jan 4, it’s really positive.3 she never had her period after she gave birth on september4 she has no contraction while she breastfeed5 she is 22 years old6 the baby in her stomach will be our second baby7 she never had a miscarriage